composed of stout rectangular feldspars, with a somewhat below the falls, and then tracing it back into the finer rounded outline, and internally changed to saussurite, though grained and more compact part near the bridge.” their periphery is mostly clear. Between these are the remains of former hornblende (possibly pyroxene) individuals now aWilliams, op. cit., p. 83. represented only by amphibole fibers and chlorite. Beautiful bIbid., p. 94. skeleton forms of leucoxene, composed of three sets of c parallel bands reproducing the rhombohedral parting of the Wadsworth, M. E., Report of State Board of Geological Survey for original ilmenite, are abundant.b 1891 and 1892, Lansing, 1890, p. 125. The rock of the schist band is much more altered. Its Origin of the schistosity.—The least-altered phases of feldspar is mostly changed to calcite, and the the fine-grained greenstones are characterized by a hornblende to chlorite. The original structure has wholly rhomboidal parting. As the rocks depart more and more disappeared, and there is now a fine mosaic of quartz from their original character the rhombs produced by the and secondary albite between the calcite and chlorite parting become more and more elongated until finally masses. The same skeleton forms of leucoxene remain, their sides are approximately parallel and a well- however, and “there is no doubt developed schistosity results. At the same time the original components of the rock mass undergo change. a Williams,, op. cit, pp. 77-78. In many of the less-altered schists that have been bIbid., p. 83. derived from massive eruptives the structure of the originals can still be recognized, though their mineral that the two specimens represent the same rock in composition can only be surmised, since they consist of different stages of alteration, the more changed form secondary products exclusively. having become decidedly schistose.”a Basic lavas and their derived schists.—In the field, dark- FINE-GRAINED VARIETIES. and light-colored schists are readily distinguished, and The finer-grained greenstones and greenstone-schists both are closely associated with nearly massive phases. were originally diabases, basalts, and basic tuffs. In On the east side of the basin, below the Little Quinnesec composition they are similar to the coarser-grained Falls, the dark schists are so intimately associated with varieties described. Their structure is less well defined, massive greenstones that there can be little doubt as to and in some cases the original rock appears to have the original identity of the two. In the hand specimen the been essentially a glass. One of the best preserved of least schistose rock is a compact, aphanitic mass of a the fine-grained rocks is that on the Michigan side of the dark-green color. Little Quinnesec Falls. It is a massive, dark-green rock Under the microscope the original diabasic nature of the rock is which, according to Williams— at once apparent, although the extensive mineralogical was originally a diabase, although its present constituents are changes which have gone on have greatly obscured its former for the most part secondary. The shapes of the original structure. The components now present are almost wholly minerals are indistinctly outlined, and so the structure of the secondary. These are hornblende, chlorite containing epidote, rock is preserved. There is now present a pale-green quartz, and leucoxene. Ilmenite and occasional traces of hornblende, probably secondary to pyroxene, although no feldspar are the only original constituents which remain. Still, traces of this mineral are preserved; epidote, chlorite, the disposition of the secondary minerals is such as to outline saussurite, and leucoxene in zones around the titanic iron. a diabasic or ophitic structure often with great distinctness. The feldspar has rarely changed to the opaque, gray The feldspar is rarely well preserved; but a narrow zone of the saussurite, but is replaced for the most part by a mass of unaltered substance of this mineral often outlines a stoutly lath- sharply defined epidote crystals. Where the feldspar and shaped crystal, even when its interior is wholly changed to an hornblende have jointly contributed to the formation of aggregate of quartz, chlorite, and epidote. The hornblende has secondary products, we have the chlorite-epidote aggregate as a curious appearance. Its crystals are brownish and a result. A little secondary quartz is also observable.b nonpleochroic with a somewhat granulated surface, so that it externally resembles diallage. Its cleavage and optical These massive or almost massive rocks, as has been properties prove it to be undoubtedly hornblende, although this said, grade in many places into schists. Where they do superficial likeness to diallage is so strong as to almost compel not actually pass into schistose phases they are often the conviction that it has originated by paramorphism from a associated with basic schists in which traces of the pyroxene. This brown hornblende is seen with a high power to original structure of eruptives can be detected or which be gradually changing to a green variety, in which a pleochroism for the first time becomes apparent. This also can be traced into rocks exhibiting this structure. In c frequently passes over into a fibrous hornblende. The chlorite- other eases, as stated by Wadsworth in connection with epidote aggregate in these rocks is very finely developed. The the discussion of the origin of these rocks at the Little chlorite is of an emerald-green color and distinctly pleoehroic. and the Big Quinnesec Falls, the basic schists may have It appears between crossed nicols as isotropic or polarizes with been derived from volcanic tuffs or, as Wadsworth calls a maroon tint. The epidote is in sharp, light-yellow crystals, them, porodites. The predominant rock on the Michigan with the characteristic shape and optical properties of this side of the lower fells, for instance, is plainly species. * * * This chlorite-epidote aggregate covers conglomeratic. It is either an old eruptive ash or an considerable areas and occupies the place of both the agglomerate. “Of the fraginental and conglomeratic feldspathic and the pyroxenic constituents. In addition to the minerals already named, ilmenite with its leucoxene border, nature of the rock,” Wadsworth declares, “no one can pyrite, and secondary quartz are quite abundant in these possibly doubt after studying it on the Michigan side, rocks.a Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 54 of 111 Through this rock passes a schistose band so intimately structure when this has not been completely destroyed. related to the massive rock in both sides of it that there Usually, however, the tuffaceous structure disappears so can not be the least doubt as to the continuity of the two. rapidly with the assumption of schistosity that nothing The hand specimen taken from the band is decidedly remains to distinguish the schists derived from tuffs from schistose. those derived from fine-grained eruptives. Under the microscope it shows the effects of mechanical In a few instances sections made from the schists at the crushing and attendant mineralogical changes with great Little Quinnesec Falls possess characteristics that point distinctness. The whole rock, with the exception of certain to a tuffaceous origin. In natural light these sections remains of the larger feldspar crystals, has been reduced to a show irregular granular areas and black dots in a fine-grained mass, showing an aggregate polarization. Light- colorless, transparent, almost amorphous matrix. green chlorite has been largely developed and has completely replaced all the bisilicate elements. The parallel arrangement Between crossed nicols plagioclase, saussurite, kaolin, of the scales of this mineral is what produces the schistose calcite, colorless or light-green chlorite, and bleached structure.b hornblende are noticeable. The plagioclase is largely altered into chlorite, saussurite, and kaolin. The greater The chlorite together with clear grains of quartz and portion of the mineral is often fresh enough to exhibit probably of unstriated feldspar are embedded in a clearly its twinning striations, though not fresh enough to quartz-albite mosaic. Calcite is also abundant as little yield accurate measurements of extinction angles. This nests in the mosaic, and rutile either in stout yellow feldspar occurs in lath-shaped crystals, in rounded and grains, or in minute sharp crystals traverse it in long irregular-shaped grains, in sharp-edged fragments, and sinuous lines. This rutile represents the ilmenite of the in some instances it appears as though it were a original rock. secondary deposit filling triangular spaces between the The mechanical action which produced this schistose band has other components. Some of the crystals are fairly well therefore resulted in the crushing of the rock, and the almost formed, others are broken, and still others are total disappearance of all of the original components. The castellated at their ends. A large number of skeleton comparatively slight change in the chemical composition of the crystals are also noticed scattered through the matrix. rock as a whole may be seen from the two following analyses * * * made by Mr. R. B. Riggs: aWilliams, op. cit., p. 91. Only a few indications of the original structures can be detected. In a few small areas of some of the sections the structure appears to have been ophitic, the plagioclase laths occurring as radial groups in a matrix of chlorite, calcite, and bleached amphibole. In most of the sections, however, no traces of diabasic structure can be detected. The rocks, though much decomposed, look as though they were composed of crystals, fragments, and small, almost dust-like grains of feldspar in a groundmass that yielded on decomposition new aWilliams, op. cit., p. 90. plagioclase, light-green amphibole, chlorite, and calcite. Most of the fragments are certainly portions of crystals bIbid., p. 90. that were crushed when the rock was made schistose, The changes here are at once seen to be due (1) to the total but there are many others which do not seem to be due removal of the iron ores (loss of iron); (2) to the production of to mashing. Some of these have concave outlines, like carbonates (gain of CO2, carbonatization); and (3) to the the outlines of splinters of minerals often found in increase in the amount of chlorite (increase of H2O, a volcanic dust. The structure thus appears to resemble hydration). more closely that of tuffs than that of lavas. These darker schists pass into lighter ones that are more CHLORITE-SCHISTS. typical chlorite-schists, composed of extremely pale chlorite and quartz grains, with a little calcite and The chlorite-schists differ from the very schistose occasional sericite shreds, and numerous minute sharp greenstones mainly in the fact that they contain no crystals of rutile. Whether these lighter schists are original plagioclase nor any saussurite derived from this. actually phases of the darker ones, or whether the two They are finely schistose, sometimes fissile, dark-green are schistose phases of rocks that were originally or light-gray rocks containing abundant quartz. The different, can not now be told, since the true nature of darker varieties are often so dark and so very fine the contact between them has been obliterated by the grained that they look very much like greenish-black changes through which the rocks on both sides of it have slates. The lighter-colored varieties resemble very soft, passed. Even if the latter be the case the two originals fine-grained sericite-schists or talcose schists. were not very different, both in all probability being basic Under the microscope the dark-green chlorite-schists are lavas or tuff beds. seen to be aggregates of green chlorite, calcite, and an Basic tuffs and their derived schists.-—The schists occasional flake of a sericitic mineral, in a mosaic of derived from tuffs generally exhibit their origin in their quartz and secondary albite. Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 55 of 111 In the light-colored schists the sericitic mineral and the greenstone is very intricately ramified by apophyses calcite are more prominent than in the darker phases. At of granite.” the same time the chlorite has been replaced by an The hornblende-schists are described as fine-grained, almost colorless variety. lustrous, black rocks, traversed by small veins of An analysis of a specimen of one of the lightest colored feldspar, as well as by cracks ramifying in various of these rocks, taken from “the western corner of the directions. They all contain more or less altered little cove just below the Lower [Little] Quinnesec Falls,”a plagioclase in considerable quantities. Some are resulted as follows: porphyrinic, others are devoid of phenocrysts. The porphyritic schists, when examined in thin section, are found— to consist chiefly of actinolite crystals so arranged as to give the rock a decided schistose structure. Distributed throughout the hornblende are phenocrysts of much-altered feldspar. These have quite the appearance of amygdules. They have altered almost completely to zoisite, with which is found also some calcite. * * * Sphene is quite abundant, some of which still retains small cores of ilmenite. The nonporphyritic schists differ from the porphyritic ones merely in the absence of the phenocrysts. They are aggregates of hornblende and comparatively fresh The small proportion of K O present in the rock would 2 plagioclase. seem to indicate that the sericitic mineral is paragonite rather than sericite. Since all of these schists contain large quantities of plagioclase they are more properly designated Between the dark and the light varieties of the schists all amphibolites than hornblende-schists. The latter are gradations are met with, the color depending upon the characterized by the possession of much quartz and but relative proportions of the dark chlorite and the colorless little feldspar. Typical hornblende-schists have not yet sericitic mineral developed. been recognized among the Quinnesec schists, though Many of the chlorite-schists of all kinds occur as definite they are found in fair abundance in the gneiss-schist bands in the schist series. They are not so related to the complex north of the Menominee trough. massive rocks as to present undoubted evidence that From the nature of the amphibolites and their intimate they are derived from these. In composition and association with the ordinary types of the greenstone- structure, however, they are identical with the most schists, it may fairly be concluded that they were derived schistose phases of the greenstone-schists described in from the same kinds of basic igneous rocks as were the preceding pages (pp. 136-139) as the end products of greenstone-schists and the chlorite-schists. Their the alteration of gabbros, diorites, diabases, and their exclusive occurrence near granite contacts would tuffs, so that little doubt can be felt as to their origin from indicate that they owe their peculiar character to their similar rocks. proximity to the intrusive. In other words, they are AMPHIBOLITES. greenstones that have been subjected not only to dynamic metamorphism, but to contact alteration as well. The amphibolites are limited in their distribution to the neighborhood of the great granite mass in Wisconsin. aLincoln, A. T., On the greenstone area in the vicinity of the Lower They nearly always occur near the contact of the granite Quinnesec Falls, Wisconsin side; a thesis submitted for the degree of B. S. in the group course in mineralogy, University of Wisconsin, 1894. with the greenstone-schists, though they are (Not published.) occasionally met with as narrow selvages along the side of granite or granite-porphyry dikes where these cut Types of rocks intermediate in character between the through the green schists. amphibolites and the greenstone-schists are found in a profusion along the banks of the Menominee River at the Williams, op. cit., p. 89. rapids known as the Horserace. Here the basic schists The only careful examination of any part of the contact are intruded by acid rocks, to which fact their peculiar between the granite and the surrounding greenstones features may be ascribed (see pp. 158-159). was made in 1894 by A. T. Lincoln. Lincoln followed this ORIGIN OF THE BASIC SCHISTS. contact about three-quarters of a mile in secs. 15 and 16, T. 38 N., R. 20 E., Wisconsin, a short distance From the field and laboratory study of the basic schists, southwest of the Little Quinnesec Falls. In an one must conclude with Williams that they are squeezed unpublished thesisa he states that “near the granite the igneous rocks, and their tuffs, which, on account of the greenstone becomes quite schistose, and in some squeezing to which they have been subjected, have lost places becomes a typical hornblende-schist. Fragments in many instances all traces of their original structure of the greenstone were broken off and included in the and nearly all of their original mineral components. granite mass at the time of its intrusion. In some parts From the nature of the products that have arisen from Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 56 of 111 the alteration of these components we are led to believe porphyroids, felsite-schists, sericite-schists, and that the original rocks were basic. In a few cases the probably paragonite-schists. The sericite-schists and original composition and the original structure of the paragonite-schists are found also associated with the schists can be reconstructed from the evidences still greenstone-schists in other portions of the southern remaining, and in these cases it may be safely asserted area. that the original rocks were gabbros, diorites, diabases, The gneisses, porphyroids, and felsite-schists are so and perhaps basalts among the crystalline varieties and similar in composition to the Wisconsin granite, which is diabase tuff among the fragmental varieties. Many of only about three-quarters of a mile from the Menominee the schists that are without a distinct igneous structure River at the Little Quinnesec Falls, that they may be are so closely associated with basic rocks which are regarded as its apophyses. Some of the larger of the almost massive, and are connected with these by such gneiss bands may be traced practically continuously into intimate gradational phases, that no doubt can be felt as the granite mass. The sericite-schists and the to their origin. These schists are unquestionably paragonite-schists in many places pass by a continuous squeezed gabbros, diorites, diabases, and other basic series of gradational phases into the felsite-schists. rocks. They are identical in every respect with many of Hence it seems probable that these are also extensions the schists whose field relations do not directly connect of the granite. them with massive eruptives, and thus lend force to the view that these latter schists are likewise squeezed Williams, op. cit., pp. 200-201. # igneous rocks. Structurally the acid rocks are in the form of dikes or The conditions under which the igneous rocks sheets. Some of them cut across the bands of basic consolidated are not plain in all cases. Some of the schists in any direction. These are unquestionably more massive greenstones were certainly originally dikes. The greater portion, however, occur in bands that dikes. Many of the others and most of the schists were trend parallel to the direction of the bands of basic probably lavas or sheets, or tuffs. Dr. Williams thinks schists surrounding them. These may be either intrusive that there is considerable evidence to show that the sheets or they may be dikes which at the present Menominee greenstones solidified at the surface under surface happen to follow the bedding planes of the schist subaerial or subaqueous conditions. bands. Below the surface they may all cut across the In the Menominee Valley this evidence consists (1) of the fine schist bands, as some are known to do. The rocks of texture of the rocks; and (2) of the alternation of bands of these two classes comprise gneisses, porphyroids, different types, which probably in their original position felsite-schists, and some of the sericitic schists. The represented successive flows. Fineness of grain is universal in finer-grained rocks are always associated with coarser- the Menominee greenstones, and we may be certain that it grained gneisses or gneissoid porphyries. A third class was a primary feature in spite of the extensive alteration of of acid schists may have been volcanic flows. These these rocks. It is especially noticeable in the case of the comprise some of the sericite-schists and paragonite- gabbro, which is almost always a coarse-grained rock when it schists. They occur in definite bands running parallel to has solidified at any depth. The succession of massive beds, the bands of the basic schists. They occur sporadically like the pale gabbros and the dark diabases seen at Lower [Little] Quinnesec Falls, are difficult to account for except by in the midst of the greenstone-schists, and are not supposing that they were once horizontal sheets which flowed associated with any forms of rocks regarded as one over another, and which were subsequently elevated into apophyses of the granite. their present nearly vertical position. Traces of tuff material are All of the acid members of the Quinnesec schists are not as distinct here as in the Marquette region, although indications of their existence are by no means wanting. We more or less schistose. The most massive granite-like might reasonably expect that any original scoriaceous or dikes, and even the peripheral portion of the granite amygdaloidal structure would have disappeared in the course boss, exhibit distinct foliation. In the porphyrinic varieties of the profound chemical changes through which these the finer-grained groundmass is very strongly schistose, greenstones have passed.a while the phenocrysts are elongated into lenticular Since the above was written undoubted tuffs have been masses. The felsites and, in general, the fine-grained discovered in the Menominee district in the vicinity of the rocks are much more distinctly schistose than the Little and the Big; Quinnesec Falls, and in Wisconsin coarser-grained ones, and many of them are fissile. about a mile southwest of the lower falls, and over a The schistosity of all the acid rocks, like that of the basic large area in the latter State several miles south of the schists, is clearly the result of pressure acting after the river, opposite the city of Norway. This discovery adds intrusive masses assumed their present position, since additional strength to Williams’s view, which must in some cases, at least, the direction of the schistosity is therefore be accepted as probably true. inclined to the direction of the walls inclosing the ACID INTRUSIVES AND THEIR DERIVED SCHISTS. intrusion. The acid rocks associated with the basic schists in Gneissoid granite and granite-gneisses.—The mass of Michigan are limited principally to the neighborhood of the rock constituting the boss in Wisconsin is— the Horserace Rapids and the Big Quinnesec Falls. a typical coarse-grained granitite, with a decided tendency to a They include gneissoid granites, porphyritic gneisses or porphyrinic structure. * * * When examined under the

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 57 of 111 microscope the macroscopic diagnosis is found to be correct, below the Horserace Rapids. The center of the band is and several additional points of interest are brought to light. a massive gray porphyritic rock. This grades on both The oldest constituents are zircon and apatite; both quite sides into a well-characterized fine-grained gneiss, abundant in the form usual in granitic rocks. Iron oxide seems resembling a granulite. The central portion of the band to be almost absent as an original constituent, though it is consists of large crystals of a zonal plagioclase found in some of the altered micas. The biotite, the only mica present, is not abundant. It is invariably bleached to a green embedded in a granular mosaic of dear, colorless, color by the reduction of its iron to the ferrous state. It contains untwinned, and probably newly crystallized grains of abundant inclusions of apatite, zircon (around which are andesine, associated with brown leaflets of biotite. pleochroic aureoles), and some secondary magnetite. No Apatite, zircon, and a reddish pleochroic sphene are also trace of either hornblende or pyroxene was observed. Sphene, present in small crystals. Calcite is also abundant. however, is present, as are also a few sharp crystals of a dark Though the rock is fresh and appears to be unaltered, grayish-blue tourmaline. The principal interest of this rock the presence of calcite indicates that it has suffered a attaches to its feldspar and quartz. They, together, make up change from its original character. The andesine is nearly the whole mass, and exhibit in a remarkable degree the a probably one of the new products formed. effects of pressure. The feldspar is of three kinds—normal plagioclase (oligoclase), unstriated orthoclase, and microcline. The gneissic portion of the band resembles the The relationship of these species of feldspar is a suggestive groundmass of the central, porphyritic phase. The chief one. Both the oligoclase and the orthoclase are always altered difference is the banded appearance of the gneiss, to a fine micaceous or kaolinitic product which is particularly abundant in the center of the crystals, a zone of the unaltered produced by the parallel arrangement of the biotite and mineral being often preserved around the edge. The the alternation of layers of different-sized grains. microcline, on the other hand, almost never shows any Tourmaline and rutile are present in some sections. indication of alteration. Remains of plagioclase phenocrysts are observed in others, but the porphyritic crystals have been nearly It is always clear and fresh in appearance, but its twinning destroyed by granulation. lamella are bent or curved and bear every sign of having been secondarily developed. * * * The large original feldspar crystals An analysis of the most massive portion of the band was show a peripheral granulation, * * * and where they have been made by Mr. R. B. Riggs. It disclosed the fact, already fissured their cracks are filled with a new crystallization of intimated above, that the rock is more nearly a diorite plagioclase, orthoclase, and quartz. None of these minerals a shows any signs of chemical alteration and microcline is never than a granite in composition. to be found among them. Thus is produced a good example of what Tornebohm has called a mortar structure (“Mörtel- Structur”). In this secondary cement-like aggregate a micropegmatitic intergrowth of quartz and feldspar is quite common, and calcite, in good-sized individuals, is by no means rare. The original quartz of this granite also shows many indications of having been squeezed. The crystals or grains often have an undulatory extinction, while larger grains are broken and the a fragments are more or less displaced. Other specimens of the granite-porphyry are more like The coarser-grained gneissic granites, augen-gneisses, granite than the one just described. In these quartz and and porphyries incorporated with the green schists are orthoclase both occur as phenocrysts. Biotite and identical in composition with portions of the great granite muscovite are present in the groundmass, the latter mass. The rocks have in many cases undergone being derived partly, at any rate, from orthoclase. The considerable chemical change, and have been subjected quartz phenocrysts are usually granulated—sometimes to a great deal of crushing. Nevertheless they can all be throughout their entire masses, often only peripherally. recognized as having differed originally from the granite only in structure. Nearly all appear to have been aWilliams, op. cit., p. 113. porphyrinic. Many of them have lost their porphyritic The augen-gneisses differ from the porphyrinic gneissic character by the crushing of their original components granite mainly in the extent of the mashing which they and the crystallization of new material between the have suffered. They are well banded or gneissic through crushed fragments. The mashing, together with the arrangement of the constituents of the groundmass, recrystallization, has changed the phenocrysts of notably biotite and muscovite, in parallel directions. feldspar into the augen of the augen-gneisses; it has Phenocrysts have disappeared by granulation, and have produced well-characterized gneisses from quartz been deformed into flattened bodies, and new porphyries; and has changed the quartz phenocrysts of crystallizations of feldspar or quartz have been the latter into granular, lenticular aggregates of this deposited in masses with triangular cross sections at the mineral. ends of the long axes of the flattened grains. The crystal aWilliams, op. cit., pp. 111-112. forms of the phenocrysts have thus disappeared, and lenticular bodies have resulted. Williams describes a band of granite-porphyry cutting the greenstone-schists on the south side of the basin, just

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 58 of 111 One of these rocks from a dike at the upper end of the to conclude that they are apophyses of this mass. A few Horserace Rapids, on the Michigan side of the river, was other schists may also be acid rocks. These are so also analyzed by Mr. Riggs. Its composition is that of a intimately incorporated with the green schists that they granitite.a appear to be integral portions of the schist series. They always occur in bands parallel to the bands of basic schists, and never cross the latter. These rocks are the light-colored white or gray micaceous schists called by Williams “silvery schists.” In composition they are mainly sericite-schists. Williams regarded them as specially metamorphosed basic schists. Some of the light schists associated with the dark-green ones may be of this character, but these are light-colored chlorite-schists. The sericite-schists are of a different character. Under the microscope they are seen to be composed of quartz grains, sericite scales, and chlorite shreds arranged in a Porphyries and felsites and their schistose phases.— parallel manner. No feldspar is noticeable in them, but The quartz-porphyries are now to all intents and calcite is abundant. As at present constituted the rocks purposes like the groundmass of the augen-gneisses, are unquestionably acid. Though no analyses are at with the addition of rounded or dihexhedral quartz grains hand to substantiate this statement, the abundance of and an occasional phenocryst of plagioclase. They are quartz and sericite noted in the slides leaves no doubt as yellowish-gray or reddish schists, with a greasy feeling. to its correctness. The components are the same as those of the augen- gneisses, but muscovite is much more abundant in them These acid schists probably represent acid flows than in the latter rocks. The mineral that was originally contemporaneous with the basic flows with which they biotite is often now represented by chlorite. All the larger are associated. Their source is not yet known. They are grains are fractured or granulated, and the smaller ones so rare that they constitute but a small fraction of the are flattened. Nests of calcite are often abundant. An Quinnesec schist series, which is essentially a set of analysis of one of these rocks by Riggs showed it to schists produced by mashing from a succession of belong unquestionably with the granite magmas.b layers of basic lavas and tuffs. aWilliams, op. cit., p. 119. INTERESTING LOCALITIES. bIbid., p. 121. Good exposures of the Quinnesec schist series and their intrusions may be seen at the Sturgeon Falls, the Big and the Little Quinnesec Falls, and along the Horserace Rapids. At the various falls only the basic members of the series are present, and a few minor bands of sericite-schists, except that at the Big Quinnesec Falls there are a few bands of gneiss. At the Horserace Rapids the basic schists are also mainly in evidence, but acid intrusions are also common. Moreover the basic schists are of a different character from those at the several falls. In appearance they are more crystalline than the latter. In composition they are more The schistose felsites differ from the schistose quartz- hornblendic and less chloritic. The peculiar character of porphyries only in texture. They are very fine-grained the schists along this stretch of the river he been aggregates of the same minerals that characterize the ascribed in previous pages (pp. 146-148) to the effect of matrix of the porphyries. Originally they may have been the granitic intrusions. felsitic apophyses of the granite mass, or possible porphyritic microgranites, that were later so thoroughly Sturgeon Falls.—The most abundant rock at the crushed that their constituents have been completely Sturgeon Falls is a coarse-grained, light-gray massive granulated into minute flattened particles which are now saussurite-gabbro. It forms the first barrier over which cemented together by newly deposited material. The the water plunges in the series of cascades that felsites as they now exist are very schistose. Muscovite constitute the falls (see Pl. X, A), and again it occurs at is abundant in them and this is always so arranged in the the lower end of the basin below the falls. The falls thin section that the long axes of its shreds are parallel proper occupy a stretch of several hundred feet along to the plane of flattening of the other components. the river. The stream here is narrow and its channel is gorge-like. Precipitous walls of the gabbro form the SERICITE-SCHISTS. banks and these are practically continuous. At the lower All the acid rocks thus far discussed are plainly younger end of the narrows the rocks are slightly schistose, but than the greenstone-schists which they intrude. From still retain the gabbroitic texture. About midway between their nature and the relations of some of them to the the upper and the lower portions of the exposure several granite mass south of the Menominee River we are led narrow bands of fissile, silky sericite-schist penetrate the Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 59 of 111 gabbro and appear midway of the east wall. Between parted joints, and gives every indication of having been pulled two of these is a narrow band of the massive rock or crushed—at all events, of having been subjected to exactly like the gabbro of the barrier. Intimate enormous mechanical strains. The joints and seams often run gradations exist between the schists and the massive in many different directions, producing a regular breccia without cement. The rock is also much slickensided, frequently gabbro. Although the gradation is much more rapid in so much so as to produce a schistose structure. The layers some places than in others, no sharp line of demarcation thus formed sometimes bend around more massive cores, can be drawn between the schistose and the massive which seem to have resisted the rubbing action. rocks. From the field relations of the two phases it is evident that the schists represent mashed and altered The formation of what are above described as “cross-gashes” forms of the gabbro. is very curious. At times the entire face of the rock wall is scarred with approximately parallel gaping seams, closely Little Quinnesec Falls.—At the Little Quinnesec Falls a resembling the rents formed in moderately dry clay or putty great width of basic schists and massive rocks, best when this is stretched. A single opening does not extend for seen in the west or Wisconsin side of the river, stretch any great distance, but a great number of them of all dimensions, closely crowded together, may produce an up and down the stream for a distance of about 1,500 irregular sort of foliation. * * * These gashes seem to have feet. The succession is made up of dark-green chlorite- been produced by a stretching of the rock or, what amounts to schists, light-colored sericite-schists, light-gray schistose the same thing, by a bulging perpendicular to the action of diabases, and several beds of fairly massive dark-green some great pressure. They resemble the “klaffende Risse,” diabase. The light schists grade into the lighter-colored described by Heim, in the Alps. * * * The edges of the seams diabases, and the darker ones into the dark diabases. are ragged, as though they had been formed by a forcible So gradual is the transition between the massive and the tearing asunder of the rock after it was solid. They are often rilled with subsequent infiltrations of secondary minerals like schistose phases of the rocks and so intimate is the a relation of the two varieties that there is no escape from calcite or quartz, but more frequently they are open. the conviction that the former are but schistose portions aWilliams, op. cit., p. 80. of the latter. At the immediate foot of the falls on the east side is an exposure of sericite-schist of the kind that Big Quinnesec Falls.—At the Big Quinnesec Falls and is supposed to have been derived from an acid bed. It is the Horserace Rapids there is about a mile of continuous a little more distinctly separable from the bands of exposure on both sides of the river. The barrier rock of diabase between which it lies than are the light-colored the falls is a medium-grained, gray, saussuritized schists farther downstream, but nevertheless it gabbro, which is in places almost massive. On the apparently grades into them. Wisconsin side of the river the pale gabbro can be seen grading into a light-green schist; and again, just below On the Michigan side of the river, near the water’s edge, the engine house, on the Michigan side of the falls, it can much of the rock is essentially like that on the Wisconsin be traced into the silvery hydro-micaceous or sericite- side, but in addition there is present just below the falls schists and into green chlorite-schists. At the lower and extending from this point to the bridge over the falls extremity of the basin, below the falls, is an exposure the thick band of conglomeratic greenstone referred to that juts into the river from the Wisconsin side. The main by Wads worth (p. 102). This is doubtless a tuff bed. portion of the exposure is composed of a green massive Sericite-schists and bands of fairly massive basic rocks rock, which is a good illustration of a diabase that has traverse the tuff. The former were regarded by suffered epidotization and chloritization. In places it is Wadsworth as dikes and by Williams as special phases crossed by bands of schist, that grade imperceptibly into of the green schists. The bands of basic rock were the massive rock. Farther up, on the Michigan side, is a described by Williams as dikes. By Wadsworth it was series of exposures of practically the same rocks. thought that they might be finer-grained and more Schistose phases are more common in these exposures massive phases of the tuff beds. On both sides of the than in the one on the Wisconsin side. It is in these river the strike of the schistosity of the schists and of the rocks that the fine intergrowths of magnetite and rutile trend of the more massive bands associated with them is described by Williamsa occur. a little south of west. The dip is about vertical. All the massive or nearly massive bands of this series consist of Horserace Rapids.—At the Horserace Rapids the rocks some form of diabase, which is usually saussuritized. are markedly different from any others occurring in the On the Michigan side of the river, however, and a little Michigan area of the Quinnesec schists. They are, distance back from its bank is a high, abrupt ridge however, like many of those near the granite mass in composed of a dark coarse-grained gabbroitic rock, Wisconsin. During times of high water the Horserace is which is uniform in composition and structure throughout a foaming, dashing cataract, rushing between two low the entire length of the ridge. Near the western end of but steep walls of dark-green rocks, crossed here and the ridge the rock is composed of a light-greenish gray there by dark pink bands. When the water is low, matrix in which lie porphyritic crystals of white feldspar shelving banks extend streamward from the bases of the and black hornblende. This is the type called by cliffs, and along these the relations of the various rocks Williams “diorite.” At its southeast end the rock is green to one another may easily be studied. and fine grained. It contains no phenocrysts. Between the Big Quinnesec Falls and the lower portion This is compact and massive in structure, but everywhere of the rapids is a short stretch of comparatively quiet profoundly seamed and jointed. It is cut by cross gashes and water bordered by dark-green massive and schistose Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 60 of 111 greenstones like those in the basin below the falls. In schistose rock is darker than the more massive phase places these are cut by bands of pink gneiss. and presents a more crystalline appearance. Other exposures consist of fibrous dark amphibolite and others of dark-green chlorite-schists. The latter often traverse the massive diorites and grade into them on both sides. In general the basic rocks along the Horserace are coarser grained, darker colored, and more crystalline than elsewhere along the river.

aWilliams, G. H., The greenstone-schist areas of the Menominee and Marquette regions of Michigan: Bull. U. S. Geol. Survey No. 62, 1890, pp. 99-101; and Neues Jahrbuch für Min., etc., 1887, vol. 2, p. 263. Of. also A. Cathrein, ibid., 1888, vol. 2, p. 151. Another respect in which the Horserace exposures differ from those elsewhere is in the presence of numerous acid rocks. These occur in bands of varying width, generally conforming to the foliation of the greenstones. The bands are sometimes composed of a massive pink granite. Usually, however, they are gneissic or schistose, and the rocks are gneisses, augen-gneisses, felsites, or other acid types already referred to in previous pages. In texture these rocks are usually coarse grained, but some of them are very fine grained. In color they vary between gray and red, pink shades predominating. Although, as has been stated, the acid bands usually trend conformably with the foliation of the greenstones, there are enough exceptions to this rule to prove conclusively that many of the bands are intrusive. On the shore of the little cove in the Michigan side of the river at the head of the Horserace, the intrusive character of one of these bands is plainly shown. Two bands of gneiss cut the greenstones. One traverses the schists. At another point farther downstream a band of gneiss ends abruptly in the green schists and sends out short apophyses into them. A few other instances of similar character may be seen at several places on the Michigan side of the rapids, but the general tendency seems to have been for the acid intrusions to follow the planes of foliation of the schists.

THE WESTERN AREA. DISTRIBUTION. The western Quinnesec schists occupy a triangular area of about 5 square miles, in Michigan, extending from about the center of sec. 15, T. 40 N., R. 30 W., Michigan, westward to the Menominee River. Where it [Plate XI. A, Horserace Rapids during high water; B, Log jam crosses the river the belt is about 3 miles wide. After in Horserace Rapids] crossing the stream into Wisconsin, it gradually widens into a broad expanse, the boundaries of which are at The rapids have excavated their channel through rocks present unknown. The Fourfoot Falls are at the of an entirely different character. Here the greater southern side of the area where it crosses the portion of the river’s banks consists of a coarse, gray Menominee River, and the old Indian village of Badwater diorite, speckled by large, and often glistening, crystal is at its northern limit. The portion of the area away from faces of dark-green hornblende, which sometimes the river is outlined by a few small, scattered exposures measure 1 or 2 inches in diameter. Occasionally biotite rising as little knobs above the soil and glacial sands. flakes are discernible in the hand specimen, and minute silvery glistening scales of talc. This mineral coats the TOPOGRAPHY. walls of all cracks and fissures in the rock and dots the The topography of the area underlain by the schists faces of the hornblende crystals. In many places the possesses no distinctive peculiarities. At its east end basic rocks have a distinctly gneissic structure. The are several small rugged knobs with exposures of Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 61 of 111 greenstones on their slopes, and at the western edge Michigan side. At many places, both at this point and along the river are other knobs, some of which are rough elsewhere in the area, the more massive phases and bare. The rest of the area is covered with glacial alternate with schistose phases in the same manner as deposits that are so thick as to produce the usual in the southern area. The schists grade into the massive assemblage of hills and hollows characteristic of drift rocks, and often no break of any kind can be discovered topography. In the river channel, however, stretches of between them. This relation suggests the origin of the dead water are interspersed between rapids and falls in former from the latter. One peculiar rock of the Upper the manner described as characteristic of the channel Twin Falls remains to be mentioned. It is observed in through the Quinnesec schists of the southern area. the smooth face of the exposure extending a little south Smooth water is more common than in the southern of east from the barrier over which the water plunges. It area, and rapids are less common, probably partly is apparently a bed of conglomerate or breccia in the because of the thicker covering of drift, and partly schists. At its extreme eastern end, where first met with, because of the fact that the rocks are more uniform in it is a band 6 feet wide composed of irregularly shaped character than in the southern, district. The Upper and and sharp-edged fragments of an aphanitic green rock in the Lower Twin Falls, situated in sec. 12, T. 40 N., R. 31 a matrix composed of small angular fragments of W., are about one-half mile apart. Each is a single chlorite-schist cemented together by a mixture of quartz, cascade, with a plunge of about 20 feet (Pl. XII, B). calcite, and chlorite (Pl. XIII, A). The large fragments Between them is a stretch of quiet water (see PI. XII, A) often match together in such a way as to indicate clearly flowing gently for the most of this distance between that they were originally portions of the same mass. banks of gravel, sand, and clay. Below the Lower Twin This was crushed into fragments and these were thrust Falls the stream is fairly rapid, but its banks are free from apart, and at the same time the matrix was forced exposures for a distance of about a mile. Here the river between them. To the west this bed splits up into leaves the schist area at a little falls and rapids known as several smaller ones, the largest of which passes over the Fourfoot Falls. The final plunge is not above 1 or 2 the brink of the falls. This peculiar conglomerate or feet in height, but back of this the stream flows over a breccia band strikes approximately parallel to the rapids for a distance of 100 yards or more, and in this schistosity of the rocks in its vicinity. It appears to be a distance it makes a fall of about 6 feet. breccia zone, produced in all probability by the crushing of the rocks which it traverses. LITHOLOGY. Unlike the schists in the Menominee River, those of the The rocks of this area are grayish green in color, and so western area are rarely cut by dikes. A few intrusions of fine grained that in many instances no texture can be diabase are met with in the area, but granitic intrusives discovered in their hand specimens. Most of them are are entirely absent. schistose, but their schistosity is not particularly noticeable until an attempt is made to break them, when The microscopical features of most of the finer-grained they tend to split easily along approximately parallel rocks of this area do not afford much evidence as to their planes like slates, and only with difficulty can they be original character. The rocks have suffered so much made to break in any other direction. In some instances, alteration that in many cases all traces of the original however, the schistosity is so perfect that the rocks structures by which their nature might have been cleave exactly like slates. On many of the exposures, identified have disappeared. The composition of the especially of the more massive phases, a typical alteration products, however, and the gradation of the ellipsoidal structure is discernible. The ellipsoids vary in dense rocks into coarser-grained varieties which still diameter from a few inches to 3 or 4 feet. There is no possess traces of structure, admit of no interpretation striking contrast between the material of the ellipsoids other than that the original rocks were basic igneous and that of the matrix between them. In both cases the rocks. Their fine grain and ellipsoidal structure proclaim rock is a dense grayish greenstone without any distinct them lavas. textural features. The matrix is usually slightly more Fine-grained greenstones and their derived schists.—A schistose than the ellipsoids, but otherwise it is like few of the fine-grained greenstones still retain their them. original structure, and this is in accord with the character In a few exposures in the northern and eastern portions ascribed to them above. One of the more massive of the area the rock is a little coarser grained. It homogeneous light-green rocks from the Fourfoot Falls possesses a structure that suggests the diabasic is described by Williams as follows: structure. It is, moreover, distinctly schistose, but its Mineralogicallv there is hardly a trace of the original rock left. schistosity is more like that of the gneisses than like that Almost colorless hornblende, pale-green chlorite, zoisite, of the slates. The rock probably represents a dike or a leucoxene, and a little calcite (all of secondary origin) are the series of dikes in the fine-grained greenstone. At the present constituents; and yet the original structure of the rock Fourfoot Falls the exposures consist of alternating bands is strikingly well preserved. When viewed with a comparatively of massive, schistose, and slaty rocks striking about N. low power, in ordinary light, the outlines of long, almost 80° W., almost at right angles to the course of the river, acicular, feldspar crystals are very apparent, in spite of the fact and yet these exposures are usually schistose on the that the substance of the feldspar itself is changed to chlorite Wisconsin side of the stream and massive on the or zoisite. These outlines of former crystals make a confused aggregate, but each individual preserves its own proper form Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 62 of 111 (idiomorphic in the sense of Rosenbusch). The angular Other schists of this vicinity are composed of spaces between the feldspars produce a typical example of the hornblende, chlorite, feldspar, quartz, and leucoxene. ophitic or diabase structure, although no trace of a diabase a Most of the hornblende is a pale-green variety, mineral remains. surrounding compact cores of a brown variety of the same mineral, from which the lighter-colored phase is believed to have been derived by bleaching. In these rocks no diabasic texture was observed. This fact, together with the presence of the brown hornblende, seems to indicate that the original rock from which the schist was derived may have been a diorite.

aWilliams, op. cit., pp. 124-125. bIbid., p. 125.

[Plate XII. A, Basin below Upper Twin Falls; B, Barrier rock at Upper Twin Falls]

This rock is traversed by “schistose bands, which show indications of having been much crushed and rubbed. Slickensides are abundant, and lenticular fragments fit into one another so as to produce an imperfect sort of foliation.” Under the microscope this schistose rock exhibits the effects of great mechanical stresses. [Plate XIII. A, Brecciated band of fine-grained greenstone at Curving and interlacing areas of pale-green chlorite and of a Upper Twin Falls; B, Ridge of dolomite, south of Lake Antoine] grayish substance (perhaps the remains of titanic iron) form the main mass of this rock. Thickly scattered through these At the Twin Falls the rocks are so much altered that no are patches of a dark-brown substance, often showing trace of their original structure can be detected. They concentric zones of a clear, transparent character. These look are usually dense, homogeneous, dark-green rocks, like opal, but their optical character shows them to be single composed of a confused aggregate of pale-green fibrous individuals of crystalline quartz. Imbedded in this material of such pronounced secondary character are fragments of hornblende, shreds of biotite, remnants of plagioclase, feldspar, which have been crushed or broken. These * * * are and grains of ilmenite. Around the feldspar remnants, less changed chemically than those in the massive rock from and occupying the space originally occupied by this which this schistose band has been derived.b mineral, is now a mass of actinolite, zoisite, and epidote.

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 63 of 111 These rocks are crossed by bands of chlorite-schists, Upper Twin Falls, just below the Cascade. The bands consisting of bright-green chlorite, finely granular quartz, are only an inch or two in width. In the field they were and perhaps some secondary albite. In addition to these supposed to be stringers from the breccia already components the schists also often contain grains of referred to (p. 161) as occurring at this place. Under the ilmenite and leucoxene, flakes of biotite, and microscope the rock is seen to be very different from occasionally crystals of tourmaline. anything else found among the western Quinnesec schists. It is plainly a very fine-grained aggregate of The contacts between the schists and the massive quartz and tiny grains and specks of some black, phases are often very indefinite. The rocks grade into opaque substance. From the fact that the bands run each other by infinitesimal variations. As described by irregularly through the greenstones, it is believed that Williams,a the first step in the production of the schistose they represent cracks that were filled from above by structure is the division of the massive rock by two material from some of the Huronian beds that must once systems of joints intersecting at acute angles. These have overspread the greenstones. joints divide the mass into rhomboidal prisms, whose cross sections are well displayed on the smooth A somewhat similar rock is described by Williams from glaciated surfaces of many of the ledges. As the the same place. It constitutes one of the schist bands schistose phases of the rock are approached, the joints alternating with the more massive greenstones. The thin become more and more nearly parallel, with the result section— that the rhombs are all lengthened in a corresponding contains irregular and angular fragments of quartz and a direction. The elongated prisms finally, by a more slightly altered feldspar of considerable size. These are severe action of the lengthening process, become very imbedded in a matrix of irregular grain, composed of chlorite, thin and much extended lenses, which produce a well- calcite, quartz, and opaque iron oxide, which is accompanied developed wavy schistosity. The process is plainly by leucoxene. The chlorite scales often have a radially dynamic, the schistosity being produced by pressure divergent arrangement around the larger included fragments of a acting at right angles to the final direction of the foliation. quartz and feldspar. Coarse-grained greenstones and their derived schists.— aWilliams, op. cit., p. 133. Although most of the rocks in the western Quinnesec The particular schist band from which this specimen was schist area are of the indeterminable character taken differs from most of the schists of the area in the described above, a few of them possess more definite fact that its contact with the massive rocks is very sharp characteristics. A few exposures are plainly porphyritic. and distinct. This leads Williams to the conclusion that it The phenocrysts were originally plagioclase, and the is not simply a schistose phase of the massive rock, but groundmass was a fine-grained ophitic aggregate that that it represents a distinct bed in a volcanic series, the may have contained some glass. These rocks must be massive rocks and the schists derived from them being classed with the basalts. Their relations to the fine- old lava flows and the schist just described being a tuff grained homogeneous greenstones can not be seen, so bed. that it can not be determined whether they are dikes or flows. ORIGIN OF THE ROCKS. aWilliams, op. cit., p. 128-129. The fine-grained texture of most of the rocks of the western area of Quinnesec schists, when considered in In a few instances coarser-grained rocks have retained connection with their composition, indicates that they are enough of their original structure to leave no doubt as to old lava flows of the general character of basalts. Their their character. Of course they are much altered, but ellipsoidal structure points to the same conclusion. In they show plainly the divergent radial structure of the Crystal Falls district, a few miles west of the diabases. Moreover, some of them still possess Menominee district, the greenstones associated with the remnants of augite grains, surrounded by light-green Huronian deposits exhibit this structure in a peculiarly hornblende in the triangular areas between the fine manner. These rocks are plainly volcanic flows, plagioclase laths. Through the hornblende are scattered since they occur in well-defined beds, many of which are little grains of magnetite, ilmenite, leucoxene, a few amygdaloidal. The Quinnesec schists seem to have crystals of rutile, and nests of calcite. The plagioclase is originated in the same kinds of rocks. They have, partly fresh, partly changed to zoisite, and partly however, suffered so much more metamorphism than changed to chlorite. the Crystal Falls volcanics that their distinctive volcanic a These rocks are nearly massive, only traces of foliation characteristics have disappeared. being occasionally observed in them. Their constituents Tuff beds may have been associated with the old lava are much fresher than those of the schists and the flows, but if so, they seem to have been in very dense greenstones. Although their relations to the latter subordinate quantity. At any rate, there is no certain rocks can not be seen it is probable that they are evidence that they ever exisited in the area, though they younger than these, and that they are dike masses may have done so. intruding them. The volcanic beds were cut by basic intrusions, and then Fragmented schists.—-The only rocks in the area that the whole complex was folded. Where the folding was appear to be fragmental are several narrow bands at the Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 64 of 111 severe the finer-grained lavas and tuffs were changed Fourfoot Falls.—At the Fourfoot Falls the rocks exhibit into fissile schists, and the coarse-grained beds into greater variety. The falls themselves are not much more schistose greenstones. Where the folding was less than a rapids over ledges of greenstone-schists. The severe all the rocks were rendered schistose, but the Chicago and Northwestern Railway bridge, on the line resulting schists still retained traces of their original between Iron Mountain and Commonwealth, crosses the structures. After the folding other basic intrusions took river at the foot of the rapids. For a few hundred yards place. These are represented by a few massive above the bridge instructive exposures can be seen on diabases met with in the area. both sides of the stream. Alternating bands of massive and schistose greenstones, striking about N. 80° W., INTERESTING LOCALITIES. outcrop on bare banks and form the rapids and several The best exposures of the rocks of this area are found at small islands in the river. On the Michigan side the the Twin Falls and the Fourfoot Falls. Both the Twin prevailing rocks are massive. On the Wisconsin side Falls are easily reached by the wagon road from Iron schists predominate. Mountain to Florence, Wis. The two falls are about half At the western end of the railroad bridge the rock is a a mile apart, but it is only in the vicinity of the cascades light-green aphanitic, massive greenstone, traversed that rock exposures are abundant. here and there by wavy bands of schistose greenstone, aClements, J. M., Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 113-135. characterized by abundant slickensides. Beyond this to the north is a narrow band of a black graphitic-looking Upper Twin Falls.—At the Upper Twin Falls (Pl. XII, A schist, and north of this another band of schistose and B), low flat exposures occur practically all the way greenstone best exposed near the water’s edge. from the highway bridge around the basin below the falls Immediately north of this exposure rises a high, sheer to a point a little above the cascade. The ledges here cliff, undermined at the base by hollows produced are fine examples of the dense, dark-colored massive artificially. This consists of a soft, black, slaty-looking, greenstones exhibiting the ellipsoidal structure. This very schistose, and quite fissile rock. In the hand structure is best seen on the flat rocks near the water’s specimen and in the ledge it bears a strong resemblance edge on the Michigan side of the river just above the to a black slate, but under the microscope it is seen to bridge. The rock is dense and presents a smooth, be a chlorite-schist. At the water’s edge, beyond the almost featureless surface. On it, however, by close cliff, the schist again appears, and north of this follow inspection, can be detected round and oval areas ledges of schistose greenstone. separated from interstitial areas by fine lines. The rock within the lines and that without them is practially alike, On the Michigan side the rock is fairly uniform in except that the former is sometimes slightly different in character. The southernmost ledge just under the bridge tinge from the latter. The round and oval areas are is a massive, dense, light-green rock. The northernmost cross sections of ellipsoids which, in this ledge, vary in ledge is a dark greenstone-schist. The intervening low size from those having a diameter of 6 or 7 inches to ledges that dot the bank are massive and schistose those having a similar dimension of a little over 2 feet. greenstones that are a little coarser in grain than the None of them have amygdaloidal peripheries. rock under the bridge. In the rocks of some of these ledges the diabasic structure is plainly discernable, but Near the falls, on both sides of the river, and at the lower in most ledges the grain is so fine that no well end of the basin below the falls, on the Wisconsin side, characterized structure is observable except in thin several bands of chlorite-schist traverse the massive section under the microscope. rocks, but they are rare. At the cascade, and on its east side, is the narrow band of fragmental greenstone referred to in a previous page (p. 161). SECTION 2. NORTHERN COMPLEX. Lower Twin Falls.—At the Lower Twin Falls the ledges The rocks constituting the Northern Complex are are larger and rougher than at the more northerly fall. gneissoid granites, banded gneisses, hornblende- The rocks are practically the same at both places, but at schists, and a few feldspathic green schists, identical the lower falls the schistose phases are rather more with some of the squeezed basic schists among the common. Here can be well seen the transition between Quinnesec schists. Mica-schists also occur in the the massive-jointed greenstones and the chlorite-schists, complex, but they are rare. They have been found only and here also, on a small scale, can be seen the actual in a few exposures in the interior of the Archean area passage of massive into schistose phases along north of the limits of the map. The granites, gneisses, shearing zones. Just below the falls, on the Michigan and schists are cut by dikes of coarse-grained and fine- side, the dense rock is traversed by a seam of quartz grained basic rocks, by small dikes and veins of aplite occupying a crack along which there has been and pegmatite, and by numerous quartz veins. movement. The rock, at the distance of a few inches DISTRIBUTION. from the vein, is quite massive; nearer the vein it becomes schistose, with a foliation inclined about 45° to These rocks occupy an elliptical area north of the the direction of the vein, and very near the vein it is a Menominee trough of fragmentals, separating it from the typical chlorite-schist. Similar chlorite-schists may also Calumet trough farther north. The complex is separated be seen in this ledge coating surfaces of joint planes. from the Huronian beds by unconformities and by basal Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 65 of 111 conglomerates which will be referred to later. Only those The gray rock passes into the pink rock by almost rocks occurring along the southern edge of the area imperceptible stages, the differences in the tints of the have been studied in detail. The rest of the area has two end members of the gradation series being due been examined sufficiently closely, however, to warrant mainly to the color of their feldspathic component. the statement that it does not differ in any essential In some instances the pink stringers are very coarse- respect from its southern periphery. grained aggregates of feldspar and quartz, like TOPOGRAPHY. pegmatites. The material of these pegmatitic veins, like that of the finer-grained ones, grades into the mass of The topography of the country underlain by the the gray rock. There is nowhere any sharp contact crystalline rocks is not unlike that of Basement Complex between the two. The red granite does not seem to be areas elsewhere. In the western portion of the area of the nature of an ordinary intrusive, but it appears small and large, rounded, glaciated hummocks of rock, rather to have the character of an impregnation, which often with precipitous sides to the south, rise above the saturated the gray granite and crystallized in certain gravels and sands of the glacial deposits. In the eastern places as patches or irregularly shaped stringers, and in part of the district, where these deposits are thinner, the other places, probably along cleavage planes, as more hills protrude higher above the drift, appearing as groups regular and definite veins. In the banded gneisses, to be of knolls with bare, fairly smooth upper surfaces, but with referred to later, the pegmatitization followed ragged and precipitous sides. approximately parallel planes, but in the gray and pink SEQUENCE OF ROCKS. gneissoid granites it took place irregularly through the rock mass. The study of the relations of the rocks of the complex to one another has not afforded any more data for Under the microscope all the granites and granitoid determining their exact sequence than studies in other gneisses of this area, whether of the pink or the gray Archean areas. In general it appears that some of the variety, are found to consist of quartz, orthoclase, gneisses and some of the hornblende-schists are older plagioclase, and biotite, and large quantities of the than most of the granites. Others of the schists are decomposition products of the last-named mineral, and plainly mashed intrusives that are younger than most of of the feldspars. The most abundant of the the granites. All of the greenstone-schists are of this decomposition products are kaolin and chlorite, but nature. The aplites, pegmatites, and some of the basic sericite, calcite, epidote, and ocher are nearly always intrusives are the youngest rocks belonging in the present in small quantity. Rutile, zircon, and apatite are complex, but even these, since they are not known to cut also present as accessories. In some sections a little through the Huronian beds, are thought to have taken micfocline is also to be found among the feldspars. It their present position before the sediments were appears in all cases to be an original component and not deposited. The latest of all the intrusives are certain a new product as in the Marquette granites. coarse-grained massive diabases and gabbros. These All the feldspars are altered. Small flakes of sericite and rocks not only occur as intrusives in the complex, but spicules of kaolin are scattered through them somewhat they are found also in the lower member of the Huronian uniformly. The microcline and the plagioclase have series overlying the Archean rocks. There is no reason undergone less alteration than the orthoclase, which in to believe that any of the rocks of the complex are many sections is so completely decomposed that no metamorphosed sediments. Most of them are clearly trace of the original material can now be detected. igneous in origin. The biotite is a green variety with a pleochroism in LITHOLOGY. yellowish-green and emerald-green tints. It is present as GNEISSOID GRANITES. small flakes and in aggregate of flakes between the feldspars and the quartz. The secondary chlorite and The most abundant rocks of the complex are gneissoid epidote are found mainly in the vicinity of the biotite, granites and granitic gneisses. These rocks differ from from which they seem to have been derived. At all one another in no essential respect. The former are events, when these products are abundant the biotite merely less schistose than the latter. Both embrace a appears to be more or less bleached. series of medium-grained to fine-grained gray and pink rocks with a granitic texture that sometimes approaches The red granites differ from the gray varieties mainly in in appearance the texture of fine-grained quartzites. The the fact that the feldspars of the former inclose large pink or red granites are usually a little coarser grained numbers of inclusions of hematite and reddish-brown than the gray ones. In the hand specimen red ocher. They also contain but little biotite and almost no orthoclase is seen to be the principal constituent. In microcline. In other respects the two rocks are addition to this there can also be detected a few grains essentially the same. The red granite appears to be a of white feldspar, a large number of quartz grains, and little more decomposed than the gray rock and is an occasional flake of mica. The gray granites appear to apparently more crushed, but otherwise there is little be almost homogeneous. Here and there through them besides difference in color to distinguish the two. are stringers and patches of pink granite, but most of the All specimens of the gneissoid granites show plainly the hand specimens are of a nearly uniform dark-gray tint. effects of mashing. Many of their components are Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 66 of 111 broken and squeezed out into lenticular masses, which quartz, epidote, kaolin, calcite, and sericite. As the give the rocks their schistosity. The peripheral portions proportion of quartz present increases, sericite, calcite, of many of the grains are granulated and in these and kaolin gradually disappear, and the rocks approach granulated portions is found the greater portion of the more and more nearly the normal hornblende-schists in secondary sericite. appearance and composition. This gradation from rocks that are unquestionably altered basic rocks to true BANDED GNEISSES. hornblende-schists may not often be observed in a The banded gneisses are composed of the same single outcrop, but the two end members of the series components as are the gneissoid granites. Secondary are united by so many gradational phases that there can products are, however, more abundant than in the be little question that they are genetically connected. It granites, and the primary constituents are usually less is very evident from the microscopical study of their thin fresh looking. At first glance these banded gneisses sections that the feldspathic hornblende-schists are appear to be made up of alternate parallel bands of gray squeezed and altered basic eruptives. It is very and pink schistose granitic material like that of the gray probable that the nonfeldspathic hornblende-schists and pink gneissoid granite described above. A close have the same origin and that they differ from the inspection of the ledges, however, shows that while feldspathic schists only in the amount of mashing to these bands are approximately parallel for short which they have been subjected and the consequent distances, they nevertheless wedge out when traced for amount of alteration they have suffered. distances of 18 or 24 inches, split up into several INTRUSIVES. narrower bands, or coalesce with other bands, forming broader ones. Often, also, narrow bands of one color The granites, gneisses, and schists are cut by veins of cut across broader bands of the other color, so that the granite, pegmatite, aplite, and quartz, and by well- rocks are actually made up of an interlacing network of defined dikes of basic rocks. veins with meshes of very unequal dimension. The long Acid intrusives.—The granite veins are of all sizes, from dimensions are parallel to the apparent banding of the a few feet to a few inches in width. Their material is rocks, which is also the direction of their schistosity, and identical with that of the red granites referred to a few the short dimensions are perpendicular thereto. pages back. The banded gneisses are thus like the mottled red and The pegmatites are coarse-grained red rocks composed gray gneissoid granites in the fact that they consist of of red orthoclases and white quartz. The feldspar is in granitic impregnations in a granitic rock. The grains that occasionally have rudely outlined crystal impregnations in the banded rock, however, followed forms and the quartz in irregular areas between these. approximately parallel courses, while in the mottled While the rock bears some resemblance to a graphic rocks they occurred irregularly. The source of the granite, the pegmatitic structure is not very pronounced. impregnating material may very well be the rock in which the impregnations took place. This may have been The aplite is a fine-grained purplish rock, in which can partially dissolved, yielding solutions which transported plainly be seen small lath-shaped crystals of red feldspar the dissolved material to situations where the conditions lying in a fine-grained matrix composed of irregular were favorable to its crystallization. grains of red orthoclase and irregular dark-gray areas of a black micaceous mineral and white quartz. The rock is HORNBLENDE-SCHISTS. quite massive, not the slightest evidences of schistosity The hornblende-schists are usually lustrous greenish- being observed in it. Under the microscope the lath- black schists, with the normal characteristics of such shaped crystals are found to be andesine. In addition to rocks. They are cut by the granites in some places. In the lath-shaped crystals there are present also many other places large blocks are found included in granite. quadrangular sections of a feldspar that extinguishes Plainly they are older than the granites, and probably differently in the four quadrants. These apparently they are the oldest rocks in the northern complex. A consist of orthoclase twinned according to the Manebach second kind of hornblendic schist exists in which the and the Carlsbad laws. These feldspars are embedded rocks are so related to the granites and gneisses that in a groundmass made up of orthoclase, quartz, epidote, they must be regarded as dikes. In some places they green chlorite, calcite, a little brown biotite, and some appear as bands cutting across the banding of the rutile. gneisses, and in others as bands conforming in strike The orthoclase and the quartz compose the greater part and dip with the lighter-colored bands of these rocks. of the groundmass. They are mainly in micropegmatitic These schists are therefore looked upon as mashed intergrowths, the quartz apparently saturating the intrusives. They differ from the schists of the first kind in feldspar. Untwinned orthoclase is also often found being duller in luster and in having a greener tinge. surrounding the quadrangular feldspar phenocrysts In their petrographical character the hornblende-schists above referred to, while the quartz occupies the same vary between normal phases composed of hornblende, position with respect to areas of micropegmatite in which quartz, epidote, and perhaps occasionally a little the quartzose constituent predominates over the feldspar, and greenstone-schists composed of feldspathic one. Between the phenocrysts and the areas amphibole, plagioclase and its decomposition products, of micropegmatite is an aggregate of quartz, orthoclase, Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 67 of 111 and the other minerals mentioned above. In this These exposures are easy of access from Iron Mountain aggregate quartz and feldspar are in small grains, the by the road that leads from this city to the so-called gold former with rounded contours and the latter with very mine that has been opened in the quartzite in their irregular ones. The other constituents are also in the vicinity. They present the usual features characteristic of aggregate, the chlorite as fairly large green flakes that Archean complexes. look as though they may have been derived from biotite, the epidote as colorless irregularly shaped grains, the rutile as tiny crystals, and the calcite as nests in the CHAPTER V. interstices between the quartz and feldspar grains. THE ALGONKIAN SYSTEM. The orthoclase of the phenocrysts and also of the General character and definition.—The Algonkian rocks groundmass is always more or less altered into kaolin- like decomposition products, the porphyritic crystals, constituting the Menominee trough, though strongly however, being much less decomposed than the grains metamorphosed, are recognized as mainly sediments. of the matrix. The greater mass of these sediments is mechanical, clastic textures usually being plainly apparent in them. Basic intrusives.—The basic dikes consist of rocks that The iron formations are largely mechanical, but with the are known comprehensively as “greenstones,” dark- mechanical material an important amount of chemical green rocks produced by the alteration of basalts, and organic material was deposited, and some of the diabases, or gabbros. The dikes vary widely in size. jaspers of the formation may be wholly chemical or Some are only a foot or two in width, while others organic. The dolomites are principally chemical or measure as much as 300 feet. The smaller intrusions organic sediments, but in their lower portions there is an are usually dike-like in form, while the larger ones are abundant admixture of mechanical débris. The more boss-like in character. Their intrusions follow the sedimentary rocks have been intruded by a few coarse- same general courses for short distances, but they vary grained and some fine-grained basic igneous rocks. The in width and have irregular contacts with the rocks latter are now usually schistose. through which they intrude. The lowest member of the Algonkian system has at its A few of the basic intrusives may be limited to the bottom basal conglomerates, which rest unconformably Basement Complex. The majority of them, however, cut upon the Archean rocks of the Northern Complex. both the rocks of this series and those of the Huronian. These conglomerates may be seen at a number of In several instances the large dikes may be traced step places along the north border of the trough. Their best- by step across the contact of the granite-schists series known exposures are those at the Falls of the Sturgeon into the quartzites of the Lower Huronian without any River, made classic by Credner, Brooks, and Irving. break in their continuity, so that in these cases there is Hence the Algonkian rocks are younger than the no question but that they are younger than the quartzites underlying schists. of the iron-bearing series. The members of the system are likewise separated from The macroscopic aspects of these rocks are those of the the overlying Cambrian sandstone by a profound ancient basic dike rocks so common in Archean areas unconformity. The Algonkian rocks are folded; the everywhere. Some of them are fine-grained green rocks sandstone is practically horizontal. The latter thus lies without any peculiarly characteristic structure, others are across the truncated ends of the eroded folds. Its lower darker-colored, medium-grained rocks with a distinct layers are formed largely of the débris of the more diabasic structure, and others are coarse-grained ancient rocks. Hence the Algonkian rocks formed a land varieties with a granular structure. These last mentioned surface for a vast period of time before the deposition of are gabbros, the others are diabases, some of which are the Cambrian sandstones. very fresh, while many are much altered. Unconformity within the system.—Within the Algonkian Under the microscope the diabases are identical in every system there is an unconformity corresponding to that in respect with the diabases cutting the Quinnesec schists, the Marquette district between the Upper Marquette and and the gabbros with corresponding phases of the basic the Lower Marquette series. This unconformity is not so intrusives in the Huronian beds (see pp. 185-186). plainly marked in the Menominee as it is in the Marquette district, but it is similar in all essential respects INTERESTING LOCALITIES. to the unconformity between the Lower Huronian cherty The rocks of this series are well exposed along the limestone formation in the Penokee district and the entire northern side of the Menominee trough. They overlying quartz-slate member of the Upper Huronian. may be seen at almost any place north of the quartzite In the Marquette district the unconformity is indicated by belt in a number of small exposures separated from a marked discordance between the upper members of each other by stretches of glacial drift. The best and the lower series and the lowest members of the upper largest exposures are in secs.. 29, 31, and 32, T. 41 N., series, and by the presence of a widespread basal R. 29 W., and secs. 1, 2, and 12, T. 40 N., R. 30 W., conglomerate in the upper series. In the Penokee where the rocks forms huge bare cliffs that are almost in district it is indicated by an erosion contact between the contact with equally large cliffs of quartzite (see PI. XIV). limestone and the quartz-slate without discordance of

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 68 of 111 bedding, and by the presence of fragments of the lower STURGEON QUARTZITE. formation in the lowermost beds of the overlying slates.a In the Menominee district the direct evidence of the The Sturgeon quartzite is so called because the principal unconformity is the presence of a conglomerate or of a rock of the formation is a quartzite which is in the same coarse quartzite at the base of the upper series position with respect to the Archean complex and the containing undoubted fragments of some of the rocks of dolomite formation of the Menominee district as is the the lower series, and the presence near the base of this Sturgeon quartzite of the Felch Mountain district with respect to the Archean series and the dolomite of that series of an iron formation made up in part of the detritus a of an older iron formation. There is also, in addition, district. indirect evidence of the unconformity in the absence DISTRIBUTION AND TOPOGRAPHY. from parts of the district of the lowest and most resistant members of the Algonkian series—the Sturgeon The Sturgeon quartzite forms an almost continuous belt quartzite and the Randville dolomite. The nonexistence on the north side of the Menominee trough immediately of the iron-bearing Vulcan formation in parts of the south of the northern Archean complex, with which it is district adjacent to the Randville dolomite and the in contact. Its most easterly known exposures are in the overlapping of the Vulcan formation by the Hanbury slate center of sec. 9, T. 38 N., R. 28 W. From this point the at some of these places give further evidence of belt continues in a general northwesterly direction to sec. unconformity. The Algonkian system is therefore divided 2, T. 40 N., R 30 W., with an average width of a little less into a Lower Menominee and an Upper Menominee than one-fourth of a mile. In its eastern portion the belt series, equivalent to the Lower Huronian and the Upper is narrow, rarely reaching a breadth of one-fifth of a mile. Huronian elsewhere in the Lake Superior region. As it passes west, however, it gradually widens out, and in sec. 2, T. 40 N., R 30 W., it has a width of a mile. The data upon which the foregoing conclusions are Here the belt turns north and the rock is folded into many based are discussed in detail in connection with the pitching folds which cause repetitions of the same beds descriptions of the several formations comprising the and a great increase in the apparent thickness and different, systems under the heading “Relations to consequently in the width of the formation. For 2 miles adjacent formations.” northward exposures of the quartzite are very abundant, but near the north line of sec. 29, T. 41 N., R. 29 W., it SECTION 1. LOWER MENOMINEE SERIES. disappears under a covering of the characteristic micaceous quartzite of the Calumet trough, believed to Succession and distribution.—The Lower Menominee belong in the Upper Menominee series. series is divided into three formations. These are, in the a order of upward succession, the Sturgeon quartzite, the Mon. U. S. Geol. Survey, vol. 36, 1899, pp. 398-405. Randville dolomite, and the Negaunee formation. Throughout the distance of 16 miles between the aIrving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of eastern end of the quartzite belt and the north line of Michigan and Wisconsin: Mon. U. S. Geol. Survey, vol. 19, 1892, pp. sec. 29, T. 41 N., R. 29 W., where it suddenly ends, the 171, 443-444, 454-455, and 472. belt is marked by an almost unbroken succession of great, bare, rugged bluffs with smooth tops and almost The formations belonging to the Lower Menominee are precipitous sides, especially to the south, cut here and observed only in the center and on the northern side of there by deep, narrow gorges, or by a series of small, the Menominee trough. On the southern side of the low ledges separated from one another by stretches of trough and around the borders of the western area of glacial sands. In only one place is there a notable gap Quinnesec schists no evidence of the existence of these between neighboring exposures. The belt should extend formations is obtainable. This may possibly be due to through secs. 34 and 35. T. 40 N., R. 30 W., and sec. 2, the thick covering of drift that blankets the rocks in these T. 39 N., R. 30 W. Ledges of quartzite are found in the portions of the district, but since ledges of the soft northeast quarter of sec. 2 and in the northwest quarter Hanbury slate are found at no great distance from the of sec. 34, but in the intervening mile and a half no borders of the schist areas, it is probable that the two exposures have been found. To the north are the usual formations are actually absent. For it is hardly credible knolls of the Archean gneisses and schists. The that two such resistant formations as the quartzite and absence of exposure may be due to the erosion that dolomite could have been so completely planed down in occurred between the Lower and the Upper Menominee these particular portions of the area as to leave no times and the consequent cutting out of the quartzite at projecting ledges above the drift, while soft slate this place, or it may be that the formation exists deeply formations nearby resisted planation sufficiently buried beneath the sands. There is a marked contrast successfully to yield ledges, especially since in the between the topography of this portion of the belt and central and northern portions of the trough the quartzite the remaining portions. Instead of the southward-facing and dolomite constitute the prominent elevations and the cliffs that are so prominent a feature of the area slates the valleys between these. underlain by the quartzite elsewhere we see here a stretch of sand plain whose surface is 100 feet or more below the level of the surrounding gneiss and quartzite hills. It is true that two small streams cross the area at

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 69 of 111 right angles to the strike of the belt, and these may have The greater number of conglomerate specimens contain lowered the surface to its present position. When we a large quantity of feldspar in their groundmass. consider, however, that the Sturgeon River, where it Sometimes, as has been stated, the matrix has the crosses the quartzite, has produced no such effect, and, composition of a recomposed granite or arkose, and at furthermore, when we note that the gneiss and granite other times its composition is more nearly that of a still exist as rugged hills, whose elevation is practically graywacke.a Whether the matrix is arkose-like or that of the neighboring gneiss and granite hills at some graywacke-like appears to depend upon the character of distance from these streams, the conclusion that the the Archean shore line against which it was deposited. quartzite did not exist at this place when the present Where the Archean rocks in contact with the topography was produced seems to be the more conglomerates are mainly granites and gneisses, the reasonable of the two alternatives. There is no direct matrix of the fragmental rock contains large quantities of evidence as to the presence of the quartzite under the orthoclase—is an arkose. On the other hand, where the sand. Archean rocks are largely greenstones, the matrix contains chlorite and the decomposition products of LITHOLOGY. plagioclase—is a graywacke. In both cases the The Sturgeon quartzite comprises conglomerates, structure is usually schistose. quartzites, quartz-schists, arkoses, graywackes, and The larger pebbles in these conglomerates possess their dolomitic sandstones or quartzites. It is cut by a few original character. The smaller ones, however, show basic dikes and by veins of quartz. plainly the effects of mashing. The quartz pebbles are CONGLOMERATES. crushed into fragments, which are sometimes separated from one another by portions of the groundmass, but The conglomerates occur only in a few places very near which are as frequently aggregated into groups of the granite-schist complex. They are well exposed on variously oriented grains, differing in outline from the the flanks of this complex near the north quarter post of original pebbles in being much elongated, often to the sec. 32, T. 41 N., R. 29 W., near the south quarter post extent of becoming almost vein-like. The particles of the of sec. 6, T. 39 N., R. 28 W., near the west quarter post aggregates are crossed by strain shadows, and the of sec. 1, T. 39 N., R. 29 W., and in the north half of sec. whole appearance of the quartz suggests pressure 8, T. 39 N., R. 28 W., in which is the famous locality at phenomena. the Falls of the Sturgeon. In all these places the rock is composed of gneiss, granite, and quartz pebbles and The smaller granite and gneiss pebbles are likewise bowlders in a groundmass that is sometimes an arkose, shattered. Their individual components occur as sharp- sometimes a graywacke, and occasionally a quartzite. edged fragments scattered through the matrix at The fragments vary in size from a few inches to a foot intervals. The plagioclase and the microcline are still and a half in diameter. Some of them are well rounded, fresh, but the orthoclase has been changed to a while others are sharp edged. The matrix in which they micaceous aggregate of secondary products, in which are embedded is sometimes a white quartzite, but particles of muscovite, kaolin, chlorite, and quartz may usually it is dark colored, being either dark red or dark be detected. gray, according to the predominance in it of red feldspar The matrix surrounding the pebbles is composed of grains or of chloritic or micaceous particles derived from isolated grains of quartz, microcline, and plagioclase in the decomposition of this mineral. The darker-colored an extremely fine-grained groundmass that has nearly matrices are also schistose, the schistosity being the composition of an altered feldspar. In the darker- generally parallel to their contacts with the granite-gneiss colored phases of the rocks there is in addition to the complex. usual decomposition products of feldspar, quite a little The character of the conglomerates varies mainly with chlorite, and other greenish alteration products of biotite. respect to the groundmass. This may have the All these components are arranged in a rudely parallel composition of a quartzite, an arkose, or a graywacke, direction, which is approximately parallel to the direction and it may either be massive or schistose. The of the elongation of the crushed quartz fragments conglomerates with a quartzitic matrix are usually derived from the quartz pebbles. massive in structure, while those with an arkose or aThe term graywacke is here used in the sense in which it has been graywacke matrix are always more or less schistose. defined by Geikie and J. Roth, and in the sense in which it has been The included pebbles are in all instances fragments of used in the description of the Survey’s “Educational series of rock gneiss, granite, basic schists, and vein quartz, identical specimens,” i. e., as a fragramental rock, the cement of which is dark with the corresponding rocks in the Archean complex. colored and of the composition of a clay-slate. See Bull. U. S. Geol. Survey No. 150, 1898, pp. 84-87. The quartzitic conglomerates are simply quartzites, like The coarser grains of the matrix evidently represent the normal rock of the formation described below, sand grains in a sediment that consisted largely of the containing pebbles and bowlders of granite, vein quartz, material from which the finer portion of the groundmass and crystalline schists, together with irregular grains of was produced. This must have been a mud or silt which the individual components of the first-named rock. was made up of the finer detritus of the Archean rocks,

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 70 of 111 and which was therefore comparatively rich in the contrary, is usually much altered, even in the largest ferromagnesian components. grains. None of the grains are as completely waterworn as those in the typical quartzite. They are usually much ARKOSES AND GRAYWACKES. more angular than the latter, like grains of sand that The conglomerates pass rapidly into nonconglomeratic have not traveled far from the coast along which they beds, whose composition is like that of the conglomerate were formed. groundmass. In some places the nonconglomeratic rock The graywackes are darker than the arkoses. They are is a red massive or schistose arkose; in other places it is always schistose and fine grained. The schistosity can a dark-gray schistose graywacke; and in still other be seen to arise from the parallel arrangement of tiny places, it is a massive or schistose light-gray quartzite, shreds and plates of chlorite and some light-colored according as the matrix of the conglomerate associated mica, and in all cases where it has been carefully with it is coarse or fine, feldspathic or quartzitic, massive examined it is parallel to the bedding. The graywackes or schistose. These rocks are interbedded with the are always finer grained than the arkoses. A few large conglomerates in comparatively thin layers. They are quartz grains are scattered through them, but the main more common in the upper portion of the conglomerate portion of the rocks consists of small grains of quartz beds than at lower horizons and constitute gradation and decomposed feldspars, spicules of muscovite, phases between the conglomerates and the typical chlorite, and green biotite, crystals and grains of quartzites. magnetite, little nests of calcite, and occasionally a small The gradations can be best seen at the Falls of the prism of tourmaline. Much of the quartz of the Sturgeon River, where there is a fairly continuous groundmass seems to be secondary, as it is in the form exposure from the conglomerate into the quartzite, of interlocking areas traversed by the micaceous through transition phases that are rocks like the matrix of spicules. the conglomerate. The rock immediately above the The schistose structure is produced in part by the conglomerate is a bright red arkose; above this is a fairly parallel arrangement of the chloritic and micaceous massive gray graywacke containing here and there little minerals and in part by original sedimentation, for there nests of pink calcite; and above this again a schistose is often noticeable in the sections alternating layers of phase of the same rock. The next layer above the finer-and coarser-grained components and layers schistose graywacke is a bright red arkose, which containing more or less of the quartz and feldspathic grades through a light-gray schistose quartzite into the constituents. normal vitreous rock. QUARTZITE. There are very few of the arkoses that are not schistose. Their feldspathic constituent furnished abundant material The major portion of the Sturgeon formation consists of for the production of micaceous products, and these, massive beds of a very compact quartzite. As a rule the under the influence of shearing stresses, readily took on quartzite is entirely massive, but in a few places it is the parallel arrangement which gave rise to the schistose. Usually the schistose phases are feldspathic schistose structure. In their present condition they are and sericitic, and they are nearly always associated with feldspathic sericite-schists. The few massive arkoses the conglomerates. In a sense they appear to be met with appear as thin beds of fairly coarse-grained gradation phases between those rocks which are always pink rocks lying between thicker beds of quartzite. The feldspathic and the vitreous massive quartzites. Hence more purely quartzitic phases contained but little they are found only in the lower portion of the formation. feldspar: consequently there was less solution of their One of the best illustrations of the schistose quartzite is constituents and therefore a smaller production of mica. the ledge separating the upper from the lower basin These rocks are therefore only rarely schistose, except below the Falls of the Sturgeon River. This schist is a along their shearing zones bounding joint cracks along very fine-grained, saccharoidal dark-gray rock with a which readjustment took place during folding. silky luster on its indistinct cleavage planes, due to the presence of some sericitic mineral in plates lying parallel In thin section the arkoses are seen to be composed of to the cleavage. Above, the quartzites pass into quartz grains and fragments of orthoclase, various dolomitic sandstones by the gradual replacement of their plagioclases, microcline, and micro-perthite, in a matrix siliceous cement by dolomitic material. of finer grains of the same substances colored red by iron oxides. Magnetite crystals, clumps of rutile, an The normal rock is a very pure quartzite, composed occasional crystal of zircon, and a few grains of epidote almost exclusively of quartz grains cemented by quartz. are the only other constituents noticeable, except certain Its material appears to have been well sorted, practically micaceous decomposition products of the feldspars. all the feldspar grains and easily decomposable These are especially abundant in the finest-grained substance having been removed from the sands before portions of the matrix, which in many places is they were deposited in their present position. composed almost exclusively of kaolin, sericite, and The rock is either vitreous or saccharoidal. It is usually quartz. The larger feldspar grains are often quite fresh, white or light gray in color, but many beds are tinged the plagioclases and the microcline being almost devoid with a faint-pink shade, and occasionally a bed is of alteration products of any kind. The orthoclase, on observed that is distinctly green. One such bed is Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 71 of 111 interleaved with ripple-marked beds of snow-white minerals, and are distributed indiscriminately in the rock phases of the rock in the small knob 1,250 paces north, without respect to its schistosity. 250 paces west of the southeast corner of sec. 2, T. 39 The schistosity, which is so noticeable in some hand N., R 29 W. specimens of these rocks, is not a marked feature of the As a rule the quartzites exhibit little evidence of mashing. thin sections. It is produced by the arrangement of a Their components are large and small rounded quartz small majority of the mica flakes in parallel positions and grains cemented together by quartz material that is the elongation of a few of the quartz grains in the same optically continuous with the grains it surrounds. Many direction. of the grains are beautifully enlarged, the added quartz DOLOMITIC QUARTZITE. often building them out into well-proportioned crystals. In the pink and white varieties of the rock no other The dolomitic quartzites are intermediate in character, as constituents are present except here and there a grain of they are intermediate in position, between the normal pyrite or a small spicule of chlorite. quartzites below them and the dolomite of the Randville formation above them. The green quartzite referred to in a previous paragraph contains, in addition to the quartz, a considerable In its upper portions the cement between the quartz quantity of light-green sericite, forming a very sparse grains of the quartzites is often dolomitic. This interstitial filling between the quartz grains Under high carbonate constituent increases in quantity as the powers of the microscope this mineral appears as a thick overlying dolomite is approached, until the rock becomes felt of spicules traversing the cementing quartz and a dolomitic quartzite and finally a quartzose dolomite. In penetrating for short distances into the quartz grains. In the hand specimen the carbonate phases may usually those cases where two grains are contiguous with no be easily distinguished from the normal quartzites by recognizable intervening cement, the line of junction their porosity. Where the rock has not been exposed to between them is marked by a very thin film of needles the weather for any great time it is quite compact, but on which, to all appearances, have been formed by its exposed surface ma} be detected numerous little pits corrosion of the quartz. The needles penetrate the that have resulted from the solution of the carbonate quartz on both sides of the junction line, destroying the cement from between the quartz grains. When the rounded outlines of the grains and producing very weathering has been more profound the rock has lost its ragged ones. The phenomenon looks very much as compactness and is now a friable mass of loosely though produced by corrosive processes—as though cohering sand grains. decomposition of the quartz had been brought about In thin sections the dolomitic quartzites consist of quartz through solutions passing between the grains and grains lying in a dolomitic cement. They present no reacting on the walls of the tiny canals through which specially noticeable features. The presence of these they flowed. Single flakes of the sencite are rocks at the top of the Sturgeon formation points to a occasionally included within the grains, and little clumps gradual deepening of the waters in preparation for the of rutile are found in the midst of the felt in the large succeeding deposition of the Randville dolomite. interstices. In the schistose quartzites the proportion of sericite is VEINS AND DIKES IN THE QUARTZITE. very large, and the crushing of the quartz grains is very In general the Sturgeon quartzite is devoid of noticeable. The rocks consist essentially of sharp-edged intersecting rock masses. However, in the close folds fragments of quartz embedded in a ground-mass of already referred to as existing in the northeast portion of sericite and very fine quartz grains. The larger quartzes T. 40 N., R. 30 W., and southwest portion of T. 41 N., R. are evidently fragments that have been rubbed together, 29 W., the quartzite is much fissured by quartz veins of producing by their attrition the finer grains that are various widths, which in some places are so numerous mingled with the mica in the groundmass. This mica, in and close together as to form a breccia of quartzite all probability muscovite, is in exceedingly large fragments in a quartz cement. Some of these quartz quantities. It occurs as tiny colorless spicules and fair- veins contain small quantities of free gold, as does also sized plates, forming a matted felt, in whose meshes are the quartzite in their vicinity. An attempt was made a tiny grains of quartz. The mica fibres penetrate the few years ago to work them, but the venture was not borders of the larger quartz grains like those in the green profitable and was therefore abandoned. quartzite described above, so that the outlines of all grains are exceedingly ragged. In this portion of the quartzite area intrusions of a dark- colored diabase are also very prominent. They occur as A very noticeable feature in the schistose quartzites at narrow dikes traversing the quartzite and the underlying the Falls of the Sturgeon River is the presence of large gneiss, and also as boss-like masses partially pieces of a brown-blue tourmaline, idiomorphic in cross surrounded by the quartzite, or as small isolated knobs section. These are found only in the mica-quartz matrix. separated from one another and from neighboring They are much larger than the components of this matrix quartzite ledges by stretches of surface free from and were evidently formed after them. They possess the exposures. characteristic sponge-like or cellular structure of contact

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 72 of 111 At one other place, near the south quarter post of sec. 6, T. 39 N., R. 28 W., the conglomerates and quartzites are cut by a very large dike of coarse-grained, black gabbro that extends across the contact between the two rocks nearly at right angles to the strike of the quartzite and traverses nearly the entire breadth of this formation. At no other places were intrusives noted in the quartzite. FOLDING. Except for a slight divergence of strike in the quartzite beds outcropping in sec. 7, T. 39 N., R. 28 W., there is no certain indication of folding in the entire quartzite belt from its eastern end in sec. 9, T. 39 N., R. 28 W., to the point in the southeast quarter of sec. 2, T 40 N., R. 30 W., where it turns north into the Calumet trough. In all this distance the beds are practically monoclinal, with dips varying between 65° and 90°. Moreover, there are nearly as many dips northward toward the granite-gneiss complex as there are southern dips away from it. In the northeast quarter of sec. 7, T. 39 N., R, 28 W., however, there is a large, bare knob of quartzite showing a slight divergence in the beds, which indicates that cross folding has taken place to some extent. The general strike of the bedding is N. 65° W. (25° N. of W.), but in the southeast portion of the knob the strike is N. 75° E. (15° S. of W.), a variation of 40° from the general strike. Furthermore, the general dip of the rock is 75° to 90° N., while the dip of the beds striking S. of W. is 70° S. These observations are not sufficient to establish the nature of the fold at this place, but they are significant in that they point to the possible existence of close folds in the series. So far as can be inferred from the distribution of the quartzite, the formation constitutes the north limb of a synclinal fold pitching westward. Its southern limb would normally appear adjacent to the Quinnesec schists along [Plate XIV. Sketch map of exposures near contact between the Menominee River, but this was apparently removed the Sturgeon quartzite and the Archean complex, in T. 40 N., R. 30 W., and T. 41 N., R. 29 W] by erosion in the interval between Lower Menominee and Upper Menominee times. THICKNESS. At the western end of the district, in the southeast In the attempt to calculate the thickness of the quartzite quarter of sec. 2, T. 40 N., R. 30 W., the quartzite turns we are met by two difficulties. The first is the northward, wrapping around the Archean complex and impossibility of deciding how much of its apparent then passing eastward into the area of the Calumet thickness is due to the duplication of beds. At one place trough (see Pl. XIV). In the 3 miles occupied in the turn we have seen that duplication is probable. Whether from the Menominee trough into the Calumet trough the there is a repetition of the same beds in other portions of margin of the quartzite is made up of three salients and the belt or not we can not be certain. A second difficulty four reentrants. The salients extend eastward into a arises from the fact that we are unable to fix definitely corresponding number of embayments in the gneiss- the upper limit of the formation. schist complex. The quartzite is evidently closely folded, the salients consisting of synclines and the reentrants of If we assume that the southward-facing cliffs which so anticlines, but the rock is so massive that dips and frequently mark the southern limit of the quartzites are strikes are not easily recognized. It is plain, however, cliffs of differential degradation, that the low ground at from a consideration of those observed, that the the base of the cliffs is underlain by the dolomite synclines pitch steeply to the west, for the points from formation, and that the exposures are monoclinal, the which the platted strikes diverge—i. e., the apexes of the thickness of the quartzite may be easily calculated at a folds—are at some distance east of the western limits of number of places. At the gorge below the Falls of the the Archean rocks and necessarily above their present Sturgeon River in the northeast quarter of sec. 8, T. 39 surfaces. N., R. 28 W., is a continuous exposure of conglomerate and quartzite beds about 350 paces in width. For 300 paces the walls of the gorge are in even-bedded Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 73 of 111 quartzite, striking N. 57° W. and dipping 75° to 83° N. 29 W. At this place the quartzite has a very regular An inspection of these beds reveals no indication of bedding, striking N. 65° W. and dipping 87° S. The beds folding. The formation in its entire width appears to are beautifully ripple marked and this ripple marking is consist of consecutive beds. The thickness so perfectly preserved that it is very difficult to believe corresponding to this width, calculated at an average dip that the beds have ever been subjected to such close of 80°, is 915 feet. Approximately the same result is folding as would be necessary to produce in them a reached from measurements made on the east side of uniform dip throughout their entire breadth. The width of the river a few hundred feet from the river bank. Here the belt measured across the strike of the quartzite from the breadth of the formation is 400 paces and its dip 70° the edge of the granite-gneiss, about 150 paces north of N. The corresponding thickness is about 1,050 feet. the most northerly ledge of quartzite, to the cliff bordering the quartzite on the south, is about 500 paces. A third estimate is based on a series of fine exposures in The equivalent thickness would be about 1,250 feet. the south half of sec. 1, T. 39 N., R. 29 W. At this place the quartzite has a very regular bedding, striking N. 65° These are the only places at which the conditions are W. and dipping 87° S. The beds are beautifully ripple favorable for the measurement of the thickness of the marked and this ripple marking is so perfectly preserved formation. It is probable that its thickness, like that of that it is very difficult to believe that the beds have ever sedimentary formations elsewhere, varies in different been subjected to such close folding as would be places, but it seems safe to presume that the above necessary to produce in them a uniform dip throughout figures represent a fair average and that its maximum their entire breadth. The width of the belt measured thickness is between 1,000 and 1,250 feet. across the strike of the quartzite from the edge of the RELATIONS TO UNDERLYING FORMATIONS. granite-gneiss, about 150 paces north of the most northerly ledge of quartzite, to the cliff bordering the The fact that conglomerates occur at and near the base quartzite on the south, is about 500 paces. The of the Sturgeon quartzite has already been mentioned, equivalent thickness would be about 1,250 feet. and the locations at which some of them occur have also been referred to. These basal conglomerates contain These are the only places at which the conditions are almost every variety of fragment derivable from the rocks favorable for the measurement of the thickness of the of the Northern Complex. Moreover, the material of the formation. It is probable that its thickness, like that of fragments exhibits the same secondary structures as are sedimentary formations elsewhere, varies in different noticed in the Archean rocks. Plainly, then, the Archean places, but it seems safe to presume that the above had reached its present condition before the figures represent a fair average and that its maximum conglomerates were laid down. Furthermore, some of thickness is between 1,000 and 1,250 feet. the Archean rocks must have been formed at great RELATIONS TO UNDERLYING FORMATIONS. depths in the earth. Therefore the Archean area must have suffered deep-seated denudation before the The fact that conglomerates occur at and near the base deposition of the quartzite. The presence of the of the Sturgeon quartzite has already been mentioned, conglomerates, then, is evidence of the existence of a and the locations at which some of them occur have also great time interval between the production of the granite- been referred to. These basal conglomerates contain schists complex and the overlying Sturgeon quartzite. A almost every variety of fragment derivable from the rocks consideration of the character of the matrix of the of the Northern Complex. Moreover, the material of the conglomerates points to the same conclusions. The fragments exhibits the same secondary structures as are matrix consists largely of the débris of granites, noticed in the Archean rocks. Plainly, then, the Archean gneisses, and basic schists; or, in other words, of sand had reached its present condition before the and silt such as would be produced by denudation of the conglomerates were laid down. Furthermore, some of Archean complex. the Archean rocks must have been formed at great depths in the earth. Therefore the Archean area must Contacts of the Sturgeon quartzite with the underlying have suffered deep-seated denudation before the rocks are very rare. They are found only where the deposition of the quartzite. The presence of the conglomerates are present. In these cases the conglomerates, then, is evidence of the existence of a schistosity of the conglomerates is discordant with that great time interval between the production of the granite- of the subjacent gneisses and schists. At most places schists complex and the overlying Sturgeon quartzite. A there is a topographic depression between the two consideration of the character of the matrix of the formations, due, no doubt, to the relative ease with conglomerates points to the same conclusions. The which the conglomerates, arkoses, and graywackes are matrix consists largely of the débris of granites, eroded as compared with the adjacent granites and gneisses, and basic schists; or, in other words, of sand schists on the one side of them and the pure quartzites and silt such as would be produced by denudation of the on the other. The quartzites on the south of the break, Archean complex. however, sometimes strike directly toward the boundary of the granite-gneiss complex. This may indicate an Contacts of the Sturgeon quartzite with the underlying unconformity between the two series. rocks are very rare. They are found only where the conglomerates are present. In half of sec. 1, T. 39 N., R.

Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 74 of 111 INTERESTING LOCALITIES. southern face is an almost vertical cliff rising about 40 feet above the bed of a small stream that separates the Although the quartzite formation is well exposed quartzite from the adjacent gneisses and schists. throughout nearly its entire extent, there are several Farther west the cliff is less nearly vertical, but it rises to localities where the exposures are of special interest, greater heights. As at this place, so nearly everywhere either because of the relations shown in them or else in this portion of the district, the quartzite and the because of the fine expanse of rock surface they exhibit. granite-gneiss complex are separated from one another The “rock dam” on Pine Creek.—One of the best places by topographic breaks in the shape of narrow valleys. at which to study the quartzitic phase of the formation is These are often but 20 to 100 paces wide, have in sec. 2, T. 40 N., R. 30 W., and secs. 31 and 32, T. 41 precipitous sides, and are usually occupied by the N., R. 29 W. (PI. XIV). Here is found the greatest channel of Pine Creek or some one of its tributaries. At expanse of quartzite occurring anywhere in the several places the streams turn abruptly from the contact Menominee district, and the wildest and most valleys and cross the quartzite bluffs, producing in them picturesque scenery. Precipices, crags, and gorges likewise narrow gorges. The best example of one of abound everywhere. The precipices are so steep and these cross gorges is about 300 paces north of the south the gorges so narrow that the snow-white bluffs appear quarter post, sec. 31, T. 41 N., R. 29 W., where the main much loftier than they actually are. Moreover, the effect stream of Pine Creek runs for a short distance across of the roughness of the country on the observer is much the strike of the quartzite between walls that rise nearly heightened because of the great contrast between it and 100 feet precipitously from the stream’s banks. This the flat sand plains and rounded sand hills over which he place is known locally as the “rock dam.” The rock on must travel to reach the quartzite area. the weathered surface of the bluffs through which the gorge is cut is brilliantly grayish white and very massive, The quartzite forms a few high, bare bluffs, with only here and there exhibiting any signs of bedding. precipitous sides and broad, flat tops, and numerous When the bedding is noticeable the beds are seen to be lower and smaller hills, often with uneven tops. From nearly on end. Two dominant sets of joints traverse their eastern limits bare bluffs of black, pink, and gray them, both of which are vertical. One set runs parallel to Archean rocks extend eastward for long distances. To the creek, i. e., about east and west, and the other is at the west the bluffs end abruptly, and a comparatively right angles to this, in places apparently following the level sand plain, dotted with low, rounded sand hills, bedding. It is the opening up of one of the former by the stretches from their bases and gradually rises into a stream that produced the gorge at the rock dam. series of smoothly rounded elevations in the distant Besides these two vertical systems of joints there is a west. marked sheeting or schistose structure to the quartzite The boundary line between the quartzite and the which dips to the west at low angles (8° to 10°). The Archean rocks, unlike this boundary elsewhere, consists structure produces layers from 1 to 2 feet across, of curves with broad crests turned eastward toward the separated from one another at intervals by joints. They Archean complex, and sharp crests turned westward are so distinct that they might easily be mistaken for toward the quartzite. The former have already been beds. This apparent bedding is no doubt due to explained as due to synclines and the latter to anticlines differential movement parallel to the axis of a large fold. in the quartzites. The rock at this place is near the top of an anticline plunging to the west. Accommodation must have taken The principal rock of the bluffs, as has been intimated, is place parallel to this direction with resulting schistosity a white saccharoidal or vitreous quartzite. This is so along the zones of rock movement. This structure must heavily bedded that only occasionally can strikes and have been produced at a time when the rock was heavily dips be observed in it. Where they can be measured loaded. The joints may have been formed later. they are found to vary in such a way as to prove the existence of folds with westward-pitching axes. The dips The quartzite is cut by many white quartz veins of all vary between 50° and 90°, and are always directed to sizes up to 20 feet in width. Some of them follow the the south or west, or to some point of the compass jointing systems in direction, while others cut the rock between west and south. The strikes vary greatly. Near irregularly. In some of these gold has been discovered. the southeast corner of sec. 2, T. 40 N., R. 30 W., the Although the quartzite phase of the formation is so well strike of a definitely bedded quartzite is N. 50° W. Near developed in this vicinity, its feldspathic phases, the east quarter post, sec. 31, T. 41 N., R. 29 W., it is N. characteristic of the lower horizons, are notably absent 45° W. Near the south quarter post of the same section except at one place. They were no doubt originally it is N. 10° E., and about one-fourth mile south of the present in the interval now occupied by the little valleys north quarter post, sec. 32, T. 41 N., R. 29 W., it is N. between the quartzite and the Archean schists. The only 15° W. However irregular these dips and strikes may remnants of these beds still remaining are to be found seem at first thought, they are nevertheless distributed in near the north quarter post of sec. 32, T. 41 N., R. 29 W. accordance with the view suggested as to the structure Here a well-defined conglomerate composed of rounded of the formation at this place. fragments of granite and gneiss, with diameters ranging One of the most striking of the quartzite bluffs is that in from the fraction of an inch to 1½ feet, lying in a dark- sec. 2, T. 40 N., R. 30 W. It is a bare, rough knoll whose gray schistose matrix, occurs in patches adhering to the Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 75 of 111 almost vertical face of a cliff of Archean gneiss. The prominent object in the landscape viewed from the south strike of the schistosity and banding of the gneiss is N. and southwest, even at distances of several miles. 15° E. That of the corresponding structures in the Before leaving this portion of the district it might be well conglomerate is parallel to the trend of the cliff, which also to refer to one other exposure which may be varies from due north-south to nearly east-west. (See considered as representing the upper members of the Pl. XIV and fig. 13.) While the variation in the strike of Sturgeon formation, though more properly belonging the schistosity of the two rocks may not in itself among the outcrops of the next higher formation, the necessarily prove the existence of an unconformity Randville dolomite. The ledges are near the northeast between them, the presence of large bowlders of the corner of sec. 3, T. 40 N., R. 30 W., and near the gneisses in the conglomerate is undoubted evidence southwest corner of sec. 31, T. 41 N., R. 29 W., about that the former rocks had been subjected to wave action 300 paces west of the nearest exposure of the normal before the conglomerate was formed. quartzite. They constitute a little cliff overhanging the The conglomerate and the gneiss are cut by a dike of flood plain of Pine Creek. The top of the elevation of dark, almost black, diabase trending about northwest. which the cliff forms the eastern escarpment is covered Across a little valley the same dike can be traced into a with sand, and both ends of the cliff face also disappear ledge of quartzite, where the contrast in color between under deposits of sand and bowlders. On the face of the the two rocks is very strikingly brought out. Again, cliff, however, the rocks are well exposed for a distance farther south, there is a large dike of the same rock of about 200 paces. They are noticeably different from trending nearly north-south. This dike is just west of the the normal quartzite, even when viewed from the road. one of the larger exposures of conglomerate, which thus They weather with a rough sandy surface and rounded lies between the igneous rock on the one side and the edges, whereas the quartzite weathers with smooth gneiss on the other, a position which has protected it surfaces and sharp edges. Moreover, they are distinctly from rapid erosion. bedded and plainly folded. When examined closely the cliff is seen to be made up of a calcareous quartzite containing a few thin cherty layers and in the upper horizons a few layers of red slate. The lowermost exposed layers are comparatively flat lying, but the upper layers are in a series of westward-pitching folds, some of which are completely recumbent. The strikes and dips are therefore very different in different parts of the ledge. Opposite the bridge over Pine Creek, for instance, the strike is N. 55° E. and the dip 45° northwest. About 50 paces south of this point the strike is N. 20° E, and the dip 20° west, and about 100 paces farther south the strike is N. 40° W., and the dip 45° southeast. The weathered surface is ridgy with alternate elevations and depressions about one-half inch to 1 inch in width, produced by differential weathering of alternate layers that are usually inclined to the bedding. Along these layers differential movement has taken place, the alternate laminæ thus becoming more or less schistose. The more schistose layers representing zones of weakness were weathered more rapidly than the others and determined the position of the depressions in the surface. These laminæ, like the bedding layers, are likewise folded into minor plications, some of which are clearly recumbent folds.

Black Creek.—About 250 paces north of the south FIG. 13.—Sketch map of exposure in north half of sec. 32, T. 41 N., R. 29 W., showing relation between conglomerates and quarter post of sec. 6, T. 39 N., R. 28 W., a small stream gneisses. Scale: 1 inch = 660 feet. known locally as Black Creek, falls in a little cascade over a ledge composed partly of granitoid gneiss and Farther south, especially in sec. 2, T. 40 N., R. 30 W., partly of conglomerate (see map, fig. 14). The gneiss is several other masses of the diabase occur. These, of the usual character. The conglomerate is made up of however, do not possess the dike form of the more round and angular fragments of the gneiss strewn thickly northerly masses. They appear as small, isolated knolls through a matrix composed of the fine detritus of the rising above the sands, except in one instance, where same rock. So nearly alike are they in general the rock forms a huge black boss-like mass in contact on appearance that at a casual glance it is difficult to the east with beds of snow-white quartzite. The parti- distinguish between the original rock and the colored knob formed of the two rocks can easily be seen recomposed phase. from the road leading to the “rock dam.” It is a Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 76 of 111 the section crosses it, measuring only about 100 paces in width. On the south side of the hill, in its eastern part, the rock in the cliff overlooking the river strikes N. 75° E. and dips 75° south, whereas the beds on the north side strike N. 60° W. and dip about vertically. Thus at the eastern or narrow portion of the bluff the variation in strike between the beds on its north and south sides is about 45°. Toward the west the strike of the southern beds turns gradually to the north, and at the west end of FIG. 14.—Sketch map of exposures on Black Creek, in S. ½ the hill its entire width is made up of a series of parallel sec. 6, T. 39 N., R. 28 W., showing relation of conglomerate to beds striking uniformly about N. 35° W. gneisses.

Associated with the conglomerate are a few beds of a banded rock that strike N. 55° W. and dip nearly vertical. This is plainly fragmental and is like the groundmass of the conglomerate. A few hundred paces west is a small ledge of white vitreous quartzite and about the same distance southwest is another of the same rock. Neither show any special peculiarity. The contact between the gneiss and the conglomerate is not well enough exposed to give any very clear idea of the structural relations existing between, but of course there can be no doubt that the latter is much younger than the former. Cutting through gneiss, conglomerate, and quartzite is a great dike of medium- to coarse-grained black gabbro. In the gneiss it appears as an ordinary dike whose upper FIG. 15.—Sketch map of exposures at and near the Falls of the surface is flush with the surface of the surrounding rock. Sturgeon River, sec. 8, T. 39 N., R. 28 W. As it crosses the conglomerate it forms a prominent knob. From this point it continues southwestward as a West half of sec. 8, T. 39 N., R. 28 W.—In a hill east of ridge of knobs, with occasional quartzite exposures on the one just described quartzite is again beautifully their flanks. exposed in well-bedded, though massive, layers striking about N. 60° W. and dipping 50° to 75° north. (See map, It is to be noted here, as at the rock dam, that the fig. 15.) This hill, like the northwesterly one, is conglomerate which remains is in a position where it was precipitous on both north and south sides. To the north protected by a mass of igneous rock. of it is the granite-gneiss complex forming an North half of sec. 7, T. 39 N., R. 28 W.—Southeast of assemblage of hillocks which in some places terminates the exposures described in the foregoing paragraph the in a southward-facing cliff and in others ends in a series quartzite forms a belt of cliffs and bluffs that extends for of isolated hillocks of comparatively slight elevation. In over a mile and a half through secs. 6 and 7 without two or three places the schist formation and the quartzite interruption except at one place where it is trenched by formation are practically in contact, being separated by a the Sturgeon River (fig. 15). Near the north quarter post distance of only a few inches. The rock on the south of sec. 7 the quartzite is intruded by the great dike of side of the contact plane is a well-characterized gabbro already referred to in connection with the conglomerate. It is exposed in a small knoll about 125 conglomerate of Black Creek. East of this the quartzite paces long and 50 paces wide at 1,400 paces north and constitutes a long, narrow hill with a very steep south 1,350 paces west of the southeast corner of the section. side overhanging the swampy flood plain of the The knoll is practically isolated from the ledge of white Sturgeon. On the north the declivity is less steep but quartzite to the south and the gneissoid-granite ledges to very rugged. The rock is the typical white vitreous the north. At one place the conglomerate and granite quartzite. It is nowhere in contact with the granite, a are in contact, but the contact line is so short and the space of about 200 paces devoid of exposures usually relations between the two rocks are so obscure that separating the two rocks. The bluff is interesting from nothing can be learned from it. The composition of the the fact that it exhibits strikes and dips so distributed as conglomerate, however, is such that no doubt can arise to suggest close folding in the beds of which it is as to its meaning. It consists largely of granite and composed. Northwest of the main bluff are several gneiss bowlders in a matrix composed of the débris of ledges near the gabbro, in which the bedding strikes N. the same rocks. 55° W. and dips 87° north. The main bluff is roughly Falls of the Sturgeon.—The granites and quartzites so triangular in outline, the western end being about 300 well exposed in the western half of sec. 8, T. 39 N., R. paces wide and the eastern end, where the east line of 28 W., continue eastward in almost unbroken ledges to Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 77 of 111 the Sturgeon River and a short distance beyond. Where in “conformable contiguity” with the granites to the north. the river crosses them occur the falls and the gorge, The author’s explanation of the phenomena is given in made classic by the discussions of Credner, Brooks, the following words: “We have here, evidently, a series Rominger, Irving, and others. The falls are over of sedimentary beds deposited on a granitic substratum, granitoid gneisses (fig. 15). Below the rapids at the foot which during the upheaval became wedged in between of the falls is an open basin of comparatively smooth the plastic granite mass, tilting, and overlapping them water flanked on both sides by conglomerates and locally so as to appear the lower beds” (see p. 75). arkoses, and below this, again, is a narrow rapids about Irving corroborated Brooks’s observations concerning one-eighth mile long, in a gorge bordered by an almost the nature of the conglomerate and agreed with him as unbroken ledge of heavily bedded white quartzite, to its signification. He found the conglomerates to striking N. 65° to 67° W., and dipping about 70° to 80° consist of granite fragments in a fine-grained sericitic northeast. There is a fairly good trail on the east side of slaty rock which at times looks like a crystalline schist. the river that runs along the side of the cliff at about 20 The granite sheet described by Rominger as interleaved feet above the water’s edge. The way is rough and with the conglomerates could not be found. The broken, but the trip over the trail gives an excellent character of the rock was clear evidence to Irving that opportunity for examining the walls of the gorge on both “we have here to do with a detritus derived by water sides of the river. Throughout the entire length of the action from the granitic and gneissic area immediately to gorge the quartzite is of the same general character, the north. The slight inclination from the vertical toward except at its north end. Here the rock is schistose and the granite which these conglomerate schists sometimes has developed in it a considerable quantity of sericite, in show is, of course, no argument against their having this respect being like the matrix of the conglomerates been deposited upon the granite as a substratum” (see on the contact with the granite. Everywhere in both cliffs p. 94). the quartzite beds are conformably stratified, no indications of folding being observed anywhere. A careful examination of the rocks exposed along both sides of the gorge below the falls will show clearly that It will be remembered that Credner described the Brooks and Irving are correct in their conclusions. conglomerates as occurring interbedded with micaceous Beginning at the falls and passing southward on the and hornblendic rocks. He mentions the existence, in west side of the stream we find at the falls themselves the midst of the granite-gneiss series, of several hundred the usual granite-gneiss complex cut by a great feet of a complex consisting of thin-bedded talcose and greenstone dike, which forms a southward-facing cliff, at sandy schists, the latter being ripple marked, a thin layer the base of which is a small patch of conglomerate. of protogine-gneiss, and three beds of conglomerate, South of this, across a valley about 80 paces wide, we each 30 feet in thickness, in which were found pebbles next find a schistose sericitic quartzite which passes of gneiss, granite, and quartzite embedded in a talcose gradually to the south into the normal white vitreous sandy groundmass. The conglomerate was thought to quartzite striking N. 57° W. and dipping 80° to 83° north. be overlain by gneiss. All these rocks he regarded as Laurentian (see pp. 51-53). On the east side the sequence is much fuller. We have again at the falls a mass of gneissoid granite, cut by Brooks, in his first report, described the same greenstone, forming a small cliff at whose base is conglomerates and the rocks associated with them as a conglomerate. The banding of the gneiss strikes N. 80° series of soft, light-gray, talcose slates, underlain by four W., forming an acute angle at its contact with the beds of conglomerate, which in turn are underlain by two conglomerate. This is filled with fragments and bowlders beds of protogine-gneiss separated by a bed of chlorite- identical in character to the gneiss at the falls. schist. Their precise age he was not able to make out, though he declared that the conglomerates separate South of the conglomerate is a narrow bed of quartzite unquestionably Laurentian beds from those that are followed in succession by conglomerate, fairly massive certainly Huronian (see p. 58). pink arkose, a gray micaceous quartzite traversed by veins of red feldspar, and another bed of pink arkose, In his second report he described the sequence at the the whole having a width of about 50 steps. The last of falls and concluded that “the structural facts, in these is well exposed on the north side of the wide basin connection with the strong lithological affinities which the below the contact. For the next 100 steps, along the schist-conglomerate series bear to the Huronian, and the east side of the basin, there are no exposures, but on its still more important fact that the pebbles contained in the south side is the eastward extension of the light-gray conglomerate are unmistakably Laurentian, leave no sericitic quartzite already mentioned as occurring on the question in my own mind but that the rocks under west side of the river. This rock forms a little point consideration are Huronian and form the base of the separating the larger basin below the fells from a smaller series at this point.” (See pp. 66-67.) one still farther south. On the east side of this southern Next Rominger described the succession of beds in the basin is a ledge of massive greenstone, and on its south vicinity of the falls with great care. The conglomerate he side is the northern edge of the great exposure of described as having a sericitic schistose matrix, identical quartzite which stretches about 300 paces southward. with certain sericitic schists that are interbedded with The strike of the quartzite is N. 65° W. and its dip 70° N. ripple-marked feldspathic beds. These he thought to be The interbedded quartzites and conglomerates to the Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 78 of 111 north strike N, 70° W., and dip 80° N. Their schistosity, the sandy detritus produced by its disintegration. The on the other hand, strikes N. 55° E. next western exposures discovered are located in a swamp, about 400 paces south, 200 paces east of the There can nowhere be discovered on either side of the north quarter post, sec. 33, T. 40 N., R. 29 W. This river any evidence of the existence of granite beds south location is 5½ miles distant from the first ledge. The of the northernmost exposure of conglomerate. All of exposures are two in number, but they are so close the rocks south of this point are unquestionably together that they may be considered as parts of a single fragmental, except, of course, the intrusive greenstone, one. The rock is a friable quartzite with a carbonate and all of them appear, without doubt, to have been cement, interbanded with a few cherty layers. The third derived from rocks like those in the granite-gneiss group of ledges is in the southeast quarter of sec. 11 complex. In the ledge some of the arkoses interstratified and the northeast quarter of sec. 14, in T. 40 N., R. 30 with the conglomerates look quite like fine-grained W. It comprises five independent exposures outcropping gneisses, but in thin section under the microscope their from the sides of the valley of the little stream that fragmental character is very plain in spite of their empties into Pine Creek at Hamilton and Merryman’s schistose structure. It is these beds, very likely, that camp No. 6, near the line between secs. 11 and 14, T. Credner regarded as gneisses and talcose schists. 40 N., R. 30 W. They extend in a line running about The conglomerate grades upward through the arkoses southeast for 800 feet, thus giving a good section across and graywackes into schistose quartzite and through the formation. A dolomitic marble and dolomitic quartzite these into the massive vitreous quartzite of the gorge. constitute the northern ledges, crystalline dolomites the During the folding of the series readjustments took place intermediate ones, and cherts the southernmost one. along a zone bordering the contact with the underlying The fourth and last exposure belonging in this belt is that granites and as a result of the consequent shearing we already referred to as occurring in the northeast quarter find the conglomerates and other rocks near the base of sec. 3, T. 40 N., R. 30 W. (see p. 193). the series schistose, while the quartzites at a greater In the country between these groups of exposures no distance from this contact are practically massive. ledges of any kind have been discovered, but a pit in the Moreover, at the lower horizon the series consists of northeast quarter of the southwest quarter of sec. 7, T. several kinds of beds varying in hardness and rigidity, 39 N., R. 28 W., and several drill holes farther west (see whereas the quartzites comprise a set of beds of the p. 405) encountered cherty quartzites and dolomites same kind. It is a well-recognized fact that belonging to the formation north of the jaspers and ores readjustments during the process of folding are much of the Appleton location. It is probable that the formation more apt to take place between beds of different continues uninterruptedly between its known characters than in a series of uniform character. occurrences, except for a possible break in sec. 34, T. 40 N., R. 29 W., The whole belt with this exception has RANDVILLE DOLOMITE. therefore been colored for the formation on the map (Pl. IX). It is quite possible, however, that in the intervals Next in order above the Sturgeon quartzite is a thick between the ledges erosion has carried away the series of beds among which dolomitic layers dolomite and that the Upper Huronian rests immediately predominate. This series is identical in character with a upon the Sturgeon quartzite. The mapping of sec. 34, T. similar series in the Felch Mountain district known as the 40 N., R. 29 W., is in accordance with this view. Randville dolomite. It is, morever, in the same geological position as the latter, and hence is regarded The country underlain by the northern belt of dolomite is as its continuation into the Menominee district It has now the valley of Pine Creek. It is a low plain, covered therefore been designated by the same name. by sand, which has been partly deposited by the creek and partly by glaciers. It was a valley long before the DISTRIBUTION AND TOPOGRAPHY. advent of the glaciers, and there is some evidence The Randville dolomite occupies three separate belts, indicating that there was a valley in this place even whose positions and shapes are determined by the before the deposition of the Cambrian sandstone. The folding to which the formation has been subjected. long-continued erosion produced by Pine Creek and its These will be referred to as the northern, central, and ancestors has reduced the area to one of low relief. The southern belts of dolomite. topography is very different in character from that of the other dolomite areas in the district, which correspond to THE NORTHERN BELT. divides, and thus are elevations rather than depressions. The northern belt lies immediately south of the belt of THE CENTRAL BELT. Sturgeon quartzite, and presumably along nearly its entire extent from sec. 3, T. 40 N., R. 30 W., to sec. 8, T. The central belt of dolomite borders the north side of 39 N., R. 28 W. Only a few exposures have been seen, Lake Antoine for a portion of its length, passes eastward but they are widely distributed through the area. The between the Cuff and the Indiana mines and north of the easternmost one is a calcareous or dolomitic quartzite Forest mine, and terminates at the bluff known as Iron outcropping on the south bank of the Sturgeon River a Hill in the northeast quarter of sec. 32, T. 40 N., R. 29 W. few hundred paces south of the center of sec. 8, T. 39 The belt is well marked in places by numerous and often N., R. 28 W. It is an obscure ledge, almost hidden by large exposures, but, as in the area just described, the Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 79 of 111 exposures are not continuous. The intervals between containing many bowlders of dolomite and dolomitic them are sometimes covered by deep deposits of soil, chert, or light-colored sandstone containing an abundant but more frequently they are occupied by the Lake dolomitic cement. In either case the inference is strong Superior sandstone, which completely blankets the that the dolomite is near at hand. Consequently the underlying rocks. Fortunately, however, there has been Randville formation is mapped as underlying the some exploratory work done along the belt, and this has sandstone in these places. shown in several places the existence of the dolomite The belt as outlined by these exposures is not known to beneath the surface. Consequently on the map the color be wider than about 870 feet at any point. This width is of the dolomite formation is made to cover a continuous reached at its eastern end, at Iron Hill, and north of the belt from the known westernmost occurrence of the Indiana mine. For a mile west of the Indiana mine it has rocks belonging in it to the easternmost known a width of not more than 400 feet, but again, in the occurrence at Iron Hill. tunnel near the west line of sec. 21, its known width is The known occurrences of the dolomite and other rocks said to be 800 feet. As a matter of fact, neither the belonging to the formation along this belt are as follows: northern nor the southern border of the belt is well A tunnel and drill hole near the quarter post between defined for any great distance. At Iron Hill, at the Forest secs. 20 and 21, T. 40 N., R. 30 W., which runs north and at the Indiana mines, the position of its southern into the hill for a distance of about 800 feet all in border is known, and for a mile west of the last-named dolomite; a group of bluffs stretching for about one-half mine it is known with close approximation to the truth. mile through the southeast quarter of sec. 21 and the But only at the Cuff mine has its northern boundary been southwest quarter of sec. 22. T. 40 N., R. 30 W., and a disclosed. Elsewhere it is buried under the Lake test pit in chert just south of these; a “horse” of white Superior sandstone. dolomite in the first level of the Cuff mine; a small ledge The belt is so narrow that it has produced little effect on of the same rock on the south side of the track of the the topography. Through most of its extent it appears at Escanaba and Iron Mountain Railroad (Chicago and or near the base of the southern slopes of a ridge of high Northwestern) about 1,000 feet east of the north quarter hills formed of the Cambrian sandstone. Near the Cuff post of sec. 27, T. 40 N., R. 30 W.; dolomites and red mine and at Iron Hill, the ledges of the formation appear slates in the shaft of the Federal exploration near the as small, isolated knolls or they are grouped together, southeast corner of sec. 22 in the same town; dolomites forming little plateaus with steep southern escarpments, and red slates in the drill holes near the Indiana mine, their northern escarpments being buried under the and in the workings of this mine; red calcareous slates in overlying sandstone. the dump piles of two test pits on the north side of the Escanaba and Iron Mountain Railroad track near the line THE SOUTHERN BELT. between secs. 26 and 27, T. 40 N., R. 30 W.; two ledges of dolomite a short distance west of the Forest mine near DISTRIBUTION. the center of sec. 25, T. 40 N., R. 30 W.; one ledge and a pit exposing the same rock near the center of the The southern and most important belt of dolomite southeast quarter of this section; and, finally, the extends all the way from the western side of Prospect numerous large exposures running through the east half Bluff, west of Iron Mountain, to the village of Waucedah, of sec. 32, T. 40 N., R. 29 W. at the eastern end of the area mapped. Where not exposed at the surface the rock has been found in mines The only stretches of any considerable length over which and test shafts and pits, so that there is a reasonable the formation is shown on the map without direct certainty that it exists throughout this distance of 16 evidence of its existence beneath the surface are: (1) miles. Where there is any doubt of its existence this is The three-fourths of a mile between the west line of sec. due to a considerable thickness of overlying Lake 21, T. 40 N., R. 30 W., and the road to Hamilton and Superior sandstone. Merryman’s camp, which runs through the center of the southeast quarter of this section; and (2), the 2½ miles The westernmost exposure is near the Hamilton mine in between the west line of sec. 23, in the same town, and the southwest quarter of sec. 30, T. 40 N., R. 30 W., but the exposures near the Forest mine in the eastern the rock has been met with in underground workings of portion of the southwest quarter of sec. 25, T. 40 N., R. exploration shafts as far west as the center of sec. 25 in 30 W.; and (3), the 1½ miles between the exposures in T. 40 K, R. 31 W. (See fig. 16.) From the Hamilton mine the southeast quarter of the last-named section and eastward the belt is well marked by numerous and often those in the center of sec. 32 in the town east. large exposures and by ledges discovered in exploratory and mining operations as far as the northeast quarter of In these intervals the surface rock is the Lake Superior sec. 18, T. 39 N., R. 28 W. Beyond this the country is sandstone. This occupies most of the areas between destitute of exposures, but about 1,175 paces north, 600 the known dolomite occurrences, and, together with the paces west of the southeast corner of sec. 17, T. 39 N., drift covering, prevents access to the underlying R. 28 W., there is a small pit in the dump of which are Huronian rocks. The sandstone, however, has been many fragments of pink dolomite. No direct evidence of test-pitted in many places, and in the bottoms of the pits the continuation of the belt beyond this point is at hand, the rock is often discovered to be a conglomerate since the workings of the Breen and Emmett mines have Monographs of the USGS Vol. XLVI – Chapters 1-5.1 – Page 80 of 111